In my area, we were not allowed to do IVs during our basic clinicals. I believe that the maximum amount of clinical hours at a time in the ED that we were allowed to do is 10 hours as a basic, 8 hours as a medic (16 hour shifts were allowed in the field though), and 12 hours as a critical care medic student. I guess every program is different.
IVs are difficult at first, but over time your technique will develop and you will learn all the tricks of IV access. I had wonderful, patient preceptors for my medic clinicals that really helped improve my technique. Usually preceptors in the ED are familiar with EMS students and are good at helping you out. Just don't expect to master IVs overnight - it takes time, patience, and practice, but eventually you will see your work pay off! I am very much a perfectionist, so it frustrated me at first with my hit-and-miss IV success....but over time, I became quicker with IVs with higher success rates. Good luck on your clinical!