Modelling could start at any age. Some make the transition as kids to adult fashion models. Ending at 25 doesn't really happen - some of the biggest models around now are older than that. Victoria's Secret's biggest name, Adriana Lima, is 31. Heidi Klum's nearly 40 and still books jobs (I think.)
Not all models are full time, it depends on how successful she is and the kind of modelling. So doing it part time to college is possible. Some models are High Fashion, which means they walk in big shows and do large campaigns like Chanel or another big name house, others are catalogue models who appear in more affordable, every day fashion catalogues and the like. As I said, really depends on her workload. A successful high end model would be busy and travel a good deal for shoots. Same goes for successful actors - and struggling actors, who'd constantly be attending auditions. It also depends on whether they're shooting back to back movies, or working on television.
I'm not sure about your last bit. Depends on how successful she is and the kind of work she is committed to. And then it depends on the work itself.