Generally you see reefs every 20% of sail area, so the first reduces main area by 20%, second by 40%, and the third by 60%. At the third reef you have effectively reached tri-sail area (old ORC rules allowed a 60% reef in lui of a tri).
It is also pretty normal for cruisers to just eliminate the first reef and go strait to the second. The wind
range difference isn't huge, and at these wind
speeds typically there is little to be gained from keeping more sail area up. Except while raceing it just doesn't matter much if you give up .5kn in boat speed if you are more comfortable.