from my own personal experience...
and looking over the adjuster life results...
You can expect to double the current mileage on your chain with a verslavy.
However, being a second or third hand bike, you can't guarantee the chain has never been changed.  But 10k is a average minimum before needing a verslavy and doubling that is a safe bet.
and you have the added insurance with the slot and pin that you'll won't drop the plunger into the gears.