Also depends on which way you are going.
If your beating into 15-20, I'd expect the apparent wind to be peaking near 25. Would not be surprised if you needed to reduce sail. If you are headed the other way with the wind on your stern, the apparent maybe sub 15, and you can probably leave everything up. Just keep an eye on things when running, or you may find yourself unhappy if the true wind climbs unexpectedly, and you need to go the other way.
Headsail or main depends on the boat. Big 150 genoa, probably that goes first. Fractional rig with hardly any jib overlap, then probably the main reefs first. Every boat is a bit different, as others have stated, a bit of experimenting will help you dial it in. Enjoy