There is no hard and fast rule for "mileage" on a chain. It all depends on how well it was maintained, and how hard the bike was ridden. (An aggressive rider will stretch out a chain faster than a smooth one.)
The chain should be cleaned, inspected and lubed every 600 miles or so for longest life.
Did you get the owner's manual? It will have the chain spec....once the tension is set properly, the test is to measure a set number of links and see if the measurement falls within a certain range.
Following from Ninja Boy's manual (2012) but I believe the specs are the same for your 2008:
Slack should be within 0.8 - 1.2 inches (20-30mm) midway between engine and rear sprockets, bottom span, with the chain at its tightest position.
For the wear limit on length, with the rear wheel elevated on a swingarm stand, remove the upper chain guard, hang a 20lb weight on the bottom run of the chain (or pull the chain tight using the adjusters) and measure 20 links of the upper run of the chain. (From middle of 1st links' pin to the middle of the 21st pin). If the measurement is 12.5 inches (319mm) or more the chain is worn out.
Since you suggested the lack of care from the prior owner, be sure to replace the chain AND BOTH sprockets.
DO NOT EVER put a new chain on worn-out sprockets or you will quickly destroy the new chain!!
I have come to terms with the possibility of dying while riding, life has a 100% death rate. I would rather live having fun and end up dead in a motorcycle crash than never do anything fun because it's dangerous and I might get killed. No one makes it out alive anyway. - From a friend on another forum, used with his permission.