Assuming you have a perfectly leak tight bag (well no noticeable loss in the time it takes the resin to gel to be more accurate), then there is no need to leave the pump running once the infusion is complete.

In some cases, leaving the pump on throughout the cure can cause pinholes or dry spots on the finished part where more resin has been sucked into the catch pot.  For cosmetic parts, there can often be a slight benefit to clamping off the vacuum feed a few seconds before the resin feed so that there is a slight excess of resin in the part and thus much less likelyhood of voids or pinholes.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

The pump won't have an issue with running for hours, it's what they are designed for. 

The Resin Infusion guide on this site recommends clamping off the vacuum line once resin starts flowing into the catch pot, but other sites recommend leaving the pump running until the resin reaches initial cure. It looks like either will work. Clamping the line prevents too much resin being drawn out of the part and avoids the risk of an exotherm in the catch pot, but if you have any leakage you are going to lose vacuum and the part will be ruined. Leaving the vacuum pump on for hours whilst the part cures is going to be harder on the pump but provides some insurance against minor leaks and presumably minimises the amount of resin left in the part (giving a stronger part for less weight as long as it doesn't resin starve)...

What do you do, fellow forum members?