The standard one won't really last a "long time" even if the engine is maintained impeccably. Sure it'll not fail as such but not only does the cam wheel wear, the chain develops slack remarkably quickly making accurate timing impossible. This can happen at well under 50,000 miles. This is the prime reason otherwise good engines lose power over time. There are a zillion Buick 215 options from the likes of Cloyes with superior duplex chains which will last as long as a good cam. (again aftermarket quality cams are known to substantially outlast the factory one)
If you've gone to the trouble of removing the timing cover and you have any doubts about the condition of anything, you can replace the oil pump (upgrade to the high capacity one), timing chain and crank seal for under £150. The timing chain is absurdly easy to replace too. It seem nuts to me NOT to do it, especially if you've bought the car in unknown condition.