Let it get as nasty as possible. Once your cycle is over, thats when you do a large water change to get out the remaining debris.
Also seeing algae coat your rock/sand/glass is when you are nearing the end as it means you have an excess ammount of NO3 in your tank.
Remember NH3 -> NO2 -> NO3
Therefore, NH3/NO2 are inversely related. So as your NH3 and NO2 go down, your NO3 will go up. When you NH3/NO2 are nil, you will have a decent amount of NO3. Algae consumes NO3, so as you build up more NO3 algae begins taking off; showing when you are on the downwards slope of your cycle.