I started CIMA after AAT and have put it on hold but only because my employers are not funding my study. I've passed the Operational Level and so have a Diploma in Management Accounts from it. I'd love to continue with but find having a full time job and trying to study in the evenings a real struggle.
I'd say you would need to study for about 8 - 10 hours per week.
The exams are quite time pressured. You think 3 hours 20 minutes is a long time but trying to get enough content in to the answers is quite difficult and the time really does fly by.
I'd say if you think it's something you really want to do then go for it and stick with it.