I don't really remember, but I think I noticed it happening pretty quickly actually. I didn't realize that was what was happening for a long time til I looked it up. And I believe it happened with his mixed grain cereal too.
Since they won't mix for you (how frustrating!) I would test it at home on the weekend. Mix it up at the time you would on work days, and then pull it out of the fridge at the daycare feeding time and check it. If it seems too watery, what about using formula instead?
I breastfed exclusively except for one very short period where I had to supplement at nine months. So I am not dissing breastfeeding. But I remember one day at the end of our cereal stage when I was thinking how glad I was to be done with cereal because it was a pain to pump just for cereal some days, all of a sudden it hit me. Duh, why didn't I just get a tiny thing of formula and use that to mix with? It would have been so much easier! And mine wouldn't eat cereal with water, so that wasn't an option. I just thought I'd mention it because I am sure it would be much easier for you.