Depending on which mission this is it could be a lot shorter. The original PLSS backpacks had a two hour air supply. The LM was powered by batteries and could only sustain life for 48-72 hours depending on configuration. If they launch and rendezvous with the orbiting CSM, they can extend their survival by several days, but there's functionally nowhere to go.

For my money the best way to go is probably in the suit, outside, and let the oxygen run out while the carbon dioxide scrubbers are still working.