From reading on here and personal experience they tend to start constantly rattling around the 30,000 mile mark, give or take.
You're looking at 2hrs+ using a proper bike mechanic, and you change the worn chain. They will hook up the new chain to the old one and feed it through whilst pulling out the old one, then rivet the chain.
It's not a job i would personally ever attempt myself, especially as it seems someone found a small chain one day, then decided to build a Fazer around it by adding bits.
The other option is an endless chain and split the engine, which should just about pay for the mechanics summer holiday.
If it's a mild rattle then i'd say there's no immediate urgency, i got mine done 'cos i've got sensitive hearing and it was like torture to me.
Once it was done the bike sounded so much better though, it'll still give a slight rattle when cold as standard but once the oil warms it'll totally go away.
http://fazerowners.yuku.com/topic/31978#.UXuylrVwrSg