In a Cessna 172 flying VFR on long final, when do I extend flaps to 10, 20, and 30 degrees?
I visualise the standard lengths of the pattern and pretend I'm simply doing them in a straight line. 

I'm sure you could figure out precise lengths or time intervals if you wanted to, but ultimately in a 172 in VFR you don't need to be exact - you just need to be fully configured and established by the usual point on final approach, as if you were completing a circuit. 

The trap many people find with a straight-in approach is they leave everything too late - downwind checks, flap, etc. Make sure you allow enough room for about 15-30 seconds between flap extension, otherwise you'll find it difficult to maintain the approach angle. 

As with everything, a good straight in approach takes practice!