Hi there,
So, I know that you aren't supposed to lift weights every day in order to allow the muscles time to rest, but does this also apply to exercises like crunches, push-ups and leg lifts? How often should I do these types of exercises?
Thanks!

the theory is you should have time off to allow the nervous and muscular system time to recover and repair.

Training more than 5 days per week is possible but only for the short term. i.e in the lead up to a major comp to fine tune the body etc. and only then with a seriously indepth nutrition program and other stuff like massages thrown in.

If your just doing it get lose some weight and get in shape there really isn't the need to do more than 5 days per week. you can but you risk burn out and injury.

It's not good to do any type of weighted exercises every day, specially if you are using the same muscle group, even isometric ones such as leg lifts, ... It is ok however to do cardio every day... You can split your routine into upper and lower body and work upper then lower the very next day...

The exception to this rule are the abs and I have no idea why you can work them every day ... Maybe a Google search would answer your question...