Solved

Best times to use Kers/boost

How should I use Kers or boost button. Should I just tap it to build up my speed off a corner? Or keep it pressed down for longer using a big chunk of boost? Basically what is the best way to use Kers boost

Never use KERS when your driver is under braking, there is just no point.
If you are close behind somebody (but more than one second) you could use it just before the DRS some to get the DRS to overtake.

As for how.much you use, it depends on how much you have and if there is anyone close to you.
If there is somebody close behind you with less KERS, you could miss lot in order to escape their attack.
If you are close behind somebody, you want to use little bursts so that you can overtake in the DRS zone.

Thanks Connor, 
But is it more effective to use a little tap of boost or to hold the boost button down?

Usually under acceleration out of the corner. It is a light tap for a few tenths of a second.