I'm in a PTA program and finished my first semester. I can't speak on an RN program, but I have a BS and this is my second career. We started with a little more than 30 and now are around 20 after the first semester. You are tested biweekly. It is just a lot of information. If you have a medical background already than it will make the program a little easier.
Holding a 40 hour week job and doing a program like this would be very difficult.
If you are motivated, have great study habits and organization you will do fine. It like the previous poster is more inline with the ADN RN. There are more prereq's req for PTA at some schools than ADN, you will have to check. I've heard some struggle more with PTA, but it is so subjective.
At our school below 75% is failing and if you get below 75% in just one class you are out of the program and must wait till the following semester to reapply. There is no probation period.
Good luck, and like I said if you put in the work and are determined you can succeed. It is truly a great field and there are tons of opportunity.