Just to let everyone know, as some people don't yet: You can NO LONGER get the agility rock by doing the low wall at the pyramid. You now get the rock by "finishing the course", and it seems any course will work. The pyramid is finished when you get the gold pyramid on the top, not when you drop down.
Sometimes it's fast, sometimes it's slow, it's all random. 1 thing I have found with the rocks (I'm finished collecting for the 5th week, but can't turn in til Saturday), is if you don't get it fairly quickly, go to something else, and when you come back it will usually be fairly quick. Might just be my luck though, seeing as the number of rocks I've gotten isn't large enough for a statistical sampling. It has kept me playing though, I have to give them that! My xp lately per week has gone up nicely because I'm spending more time online. (Plus I'm close to 99 herby, but growing the herbs for it, so takes forever!)
And now for the answer to the question I know you are all asking... YES! You do still get rocks after you complete your statue, unless of course you opt out with the museum guy. I still haven't decided yet.
Week 31 of statue done... and no plinth, no statue piece, just the xp for turning the rocks in. But do keep in mind, that is a considerable amount of xp along the way!