For better or worse, creative projects can take up a lot of time. Younger kids will have a limited attention span, but as they get older, the amount of time kids can spend on art, music, or other creative projects is almost endless. For this reason, it is nice to allow time, to get into the zone (flow), develop a concept, and work through various solutions.
On the flip side, however, don't not start a creative project because time is limited. If you have supplied handy, there's no reason not to pull out the paints 30 minutes before soccer practice for example. Sometimes ideas and project marinate better with some time away. Try to be flexible, enjoy the making, and give it time - whether in one session, or several.