I try to pump for at least 20 minutes at a time. However, I just go by how my body feels. Sometimes I need to take it off after ten minutes, sometimes I've left it on for 45 (with a light pump). Start out slow, and learn to read your body. Keep an eye on the little guy and make sure he doesn't go too purple.
I've never hurt myself pumping, and have been doing so for a year now (although I unfortunately stopped for a few months because of a relationship). Cis guys who get good results do it for years in order to keep their gains, but after a few years, it should be permanent. I bought a clit pump with 3 size tubes, and have actually had to buy the next size up separately. I'm hoping to stay on track so I can go up a size again (which will put me into small penis pump sizes). I don't know why more transguys don't pump; at the very least, it helps me get a harder erection when I'm horny. Although I really do believe it has helped my length and girth get to where it is today.