The generally specified amount is about 1/3 of a cup of yeast slurry into a 20 litre/5 gallon batch - so maybe 25ml, more?. This is all very rough, because you can never be sure of the concentration or viability of the yeast without putting it under a microscope. It's difficult to pitch too much yeast at the home brew level though, so I'd err on the side of more yeast.
If a "St-Peters cream" stout is a high-alcohol and therefore hard-work environment for the yeast, it may not be very healthy after fermentation. So if you just fermented a super-double-mega-imperial-table-beer, then maybe don't re-use the yeast.
As long as you can maintain a sanitary environment, there's no reason not to re-pitch healthy yeast. If your cream stout tastes weird at all, I would certainly not re-use the yeast then. Taste the beer before pitching its yeast.
I believe the "amount of time" you are referring to, is the post-pitch "lag phase" where the yeast is initially multiplying (and creating the bulk of the esters/flavours), before fermentation becomes noticeably vigorous. Pitching a good amount of healthy yeast will reduce this time.