Tricky question, I’ve not yet completely figured it out myself! It depends on a lot of factors including not just wind strength but also gustiness and desire for comfort, safety margin etc.
In very gusty conditions eg most lake sailing, I prefer to reef early and be a little under-powered sometimes but have the boat feel great during the gusts, than to be over-powered and trying to spill the main in the gusts. When single handed I reef earlier too, also if I have inexperienced crew on board.
For me though, I’d say 15mph of wind. This is when whitecaps start on the waves, so I also reef if there’s any sign of whitecaps.
Don’t forget that many capsizes are caused by genoas – for higher winds it’s just as important to get a smaller jib up as to reef the main, maybe more important as it’s harder to spill air off a jib by easing the sheet.