I couldn't agree more with some of the points that Polarbear raised wrt the time of day, ability to focus and conflicts with an existing homework load.
I found this out myself the hard way with my DS8 - I just couldn't understand why sometimes she would mentally attend to conversations, ideas etc and other times act like the World's first surviving brain donor. Eventually, yes I was SLOOWWWWW, I realised that there was a pattern and that pattern was that right after school she would need a bit of a break to 'process' the events of the day before she could re-engage. Finding the sweet spot is a bit of an art because leave it too long and then fatigue sets in anyway because it is too close to bedtime.
Homework that needs to be done also needs to be factored in and this is actually why DW and I met with the school last week - after school enrichment may not be sustainable once homework load ramps up.
All in all, based on our experience so far, I think that ability/level appropriate work is best done during the hours when the child is most alert - the school day.
