Best time is before it breaks!!

But seriously my 1998 helms recommends 105k miles, I think Honda upped their old suggested 60k miles. I would still suggest 60k miles if you drive somewhat aggressive. At the same time replace the waterpump, change the coolant (it needs to be drained anyways), replace all the all other drive belts (these need to be taken off anyways), adjust the valve clearance(valvecover needs to come off anyways), and inspect the cam seal (replace if there is a slight hint of oils seepage).

Pro-tip: Buy yourself a helms manual and do-it-yourself!!