READ!!!

If we're talking about a truly realistic scenario, depending on the ant they can live anywhere between half a year up to 15 years. But at the end of the day without food or water, Marines will all die in a couple weeks. Given no food requires no matter what, Marines could easily hold out for 15 years. All of the remaining ants would be dead far before they could overrun a googolplex of Marines. Case solved.

Sigh... read the rules.

If we're using realism the whole thing collapses immediately into a singularity.