Gear box oil, nsr 125

[Carlos NSR125]

Hey, I have recently just bought an 02 plate NSR 125 it had 3500 on the clock now its on 4800.
But I was just wondering when the gear box oil needs to be changed??
If it starts to make a *clunk* noise in every gear am guessing that it need's doing. But I'm just a bit unsure when I will need to do it, because when I look at the level mark in the little window, it looks like it has some in but I can't tell.
Any help would be great thanks
Carlos
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CBT Passed 11-04-09 Honda NSR 125 3-04-09
New Tyres 30-6-09