I've just finished my placement matches. Got high enough to be happy and don't know how often I'll play ranked. How fast does elo decay happen. If I didn't play ranked for a month what would my elo be? Anybody know? Right now it is 1398

I think elo doesn't decay under 1400.

i read that elo decay has been disabled until further notice

Your current ELO will decay if you are over 1400 after 4 weeks of inactivity in the respective queue, down to 1400, gradually. I think it's ~24 ELO a day either till you play a game or reach 1400. You can't decay below 1400.

Your highest ELO will remain that which you are awarded for.

So Ervian is right, your current ELO will not decay, as it is not over 1400.