It's technically not a "bad" thing to change pets occasionally but it can definitely become time consuming if done often. It's probably better to not get into the habit of replacing your pet, unless there's a rare beastie that you feel you just have to get, which you were too low level for earlier. It pays to visit Petopia [linked below] and plan ahead.
http://petopia.brashendeavors.net/
I haven't timed exactly how long training points take to gather up but the time required just gets longer as you train higher level pets. I've had about six or seven different pets over the course of one of my hunters. But they were mostly had during the lower levels, to experiment with the feel of playing with different species.
If you're interested in trying out the kind of wait you might be in for, stable your current pet, train something new and run around doing your quests, feeding the new pet as it needs it. Then you'll see how long fresh training points take at level 20 and can remember that it gets gradually slower from then on.