Hello, I'm level 15 and never played pvp (sad isn't it) and I was wondering if the recommended builds were any good or is it just a matter of opinion and stuff I've been playing Volibears recommended build that the game tells me to get and I do fairly well (against bots) but when I try any other build it seems it takes longer or not as effective as I would like (going for huge tank builds). Is it depending on player or am I just that bad?

The recommended builds are pretty decent nowadays. They are a good guideline to go on until you get to know the game and the champs you play. By lvl 15 however you should be researching up on specifics and how they work on champs. I would say go to Solomid.net and check their guides out for builds runes and masteries plus quite often they have video guides on how to play that champ.
GL to you!

It depends on what you're going for. If you're reaching for all expensive items, then it's going to take longer to build. You won't always complete your build every game, in fact you probably won't in the majority of your games. The best thing you can do is have a core build you generally stick to that works, then get items that work with your team, or counter the enemy team.

Recommended builds are good for starting out, but it's best to start experimenting with items. Volibear works well with a lot of health, so getting something like a Warmog's as a core piece is good. A lot of people get something like Wit's End or Ionic Spark to complement his bite and his ultimate. Do remember though, build magic resist and armor, not just pure health. It may seem like a good idea, but if a ranged AD gets a hold of you with any amount of lifesteal and damage, you're gonna have a bad time.

The recommended builds are okay, but they're not the best and they're more of a guideline. At your level, I'd absolutely recommend you go check out mobafire.com. They tend to have the most thorough guides to champions and tell you piece by piece how to build you champ. Understand, of course, that this is more of a starting place than anything. Try using the mobafire builds, but then always feel free to change them depending on the situation and experiment with what you think works better.
With Voli for example, a build might tell you to build a lot of armor, but if you're up against a whole bunch of ap carries, you'll want to buy magic resist instead. And if you're having trouble getting items fast enough, heart of gold is a great item on Voli. It gives him a nice health boost, and gives you extra gold per 10 seconds so that you can get gold for those big ticket items sooner.

The recommended builds don't really cover the roles or steps. Like most supports start mana pendant, a couple wards, a couple potions, build gp10 items (philospher's stone, heart of gold), aegis, shurelya, while the recommended build will show (looking at soraka's now: regrowth pendant, ionian boots, kindle gem, shurelya, aegis. Basically it leaves out a lot of steps that beginner's wouldn't know even if it gives a decent final build.

Part of it might just be the role though. I don't play volibear but I assume he's most effective as support in a double lane, tossing opponents behind him for his teammate to beat up on. So playing with people for the first time, perhaps where you're losing it is the teamwork aspect, and not the build. I play solo queue supports a lot, if my "carry" partner can't last hit well or positions poorly and is too aggressive, I know we're in trouble, and my build isn't going to help.

I think the consensus is that the recommended builds are occasionally decent and mostly terrible. Even when they are decent, they fail to account for situational picks like order of build, countering enemy team, falling behind on gold and difficult match-up. Not to mention that consumables like potions and wards are never recommended despite being important throughout the game.

On the other hand, the hard part of trying to follow a build on a guide when you are level 15 is that the said guide will often make assumptions about how well you can farm at various phase of the game. For example, some builds for ADC will go straight from boots to big items like IE, but if you have yet to master farming, you will end up spending 20+ minutes trying to get those items while being essentially under-equipped.

To do this properly, you should take some time to do a bit of research about your champion, your role and the common items used for that role. This is where the guides are useful, because the good ones will go into details about when and why they consider one item to be better than the other item. Another useful thing is to watch live streams, because you can see how different champions' effectiveness change as they grab different items.

The correct solution to playing poorly is to get better at the game, but how is this tautology going to help new players?

For the benefit of the OP, if a guide is asking you to build items in certain sequence, and you are having problems farming for those items, then it is important to have in-between items, like Doran's or GP items so you can either keep up with power or keep up with gold. Sometimes you need to read these contingencies into guides. The more helpful guides will explain some of these situations, but not all of them will. Again, it is worth the time to look a bit deeper into this when you are doing your research.