When to reef-or reduce
As the replies have indicated, this is not a cut and dried question. When the boat needs less sail is pretty easy to tell. If you are in cruise mode (without a bunch of guys on the rail), you will want to reduce some sail by the time you have about 12 knots True Wind speed (this is upwind of course-you can handle more wind at deeper sailing angles before becoming overpowered).
Notice I said REDUCE, not reef. Assuming you were starting with a 150% genoa, then by the time you have 12 knts TWS (about 17-18 apparent WS) you really want less sail-you can either roll up some genny, go to a smaller headsail, reef, or both.
I would say that if you use a 110% genny, you should be fine upwind up to about 14-15 TWS-after which a first reef is a good idea.
These are all ballpark windspeeds-you will need to determine this for your own boat-and even among same models the point will differ based on how the boat is loaded, etc.
It is safe to say, though, that by the time you have the rail in the water, or have more than 1/4 turn of the wheel needed to keep her straight-you need to reduce some sail. I suggest anything over about 15 degrees of heel as a "setpoint" -since anything much beyond that is slower anyway.
On almost all modern boats, the second reef will hurt performance much more than going to a smaller headsail-it may not be practical to do otherwise, but I wanted to put that out there-the second reef is certainly OK to do, but you are much less efficient.
So, when to reef cannot really be answered until we know what size genoa you are using, and whether you can partially furl it.
Cheers!
S