How many times can you use your murder servitors before having to replenish them?

As often as the GM feel fit....
I would consider the on board techpriests repairing them, sacrificing less interesting crew members to refit the mechanica etc.

Like everything else, making your acquasition check includes basic maintinance and recovery of consumables. When the GM feals the item has been 'exceptionaly damage' s/he can call for a Maintinance roll to keep it from being lost or reduced in quality. So yeah, untill the GM says so.