Equal to your speed.
Not trying to be snide; it really is. From the Movement section:
Most characters and monsters have a speed statistic—also called land Speed—which indicates how quickly they can move across the ground. When you use the Stride action, you move a number of feet equal to your Speed. Numerous other abilities also allow you to move, from Crawling to Leaping, and most of them are based on your Speed in some way. Whenever a rule mentions your Speed without specifying a type, it’s referring to your land Speed.
As pointed out by ESCE, the Soaring Rune:
A set of soaring armor helps you fly faster and protects you and nearby allies from falling. While wearing soaring armor, you gain a +10-foot item bonus to your fly Speed, if you have one. As normal, if your fly Speed is based on your land Speed and you already have an item bonus to your land Speed, these bonuses aren't cumulative.
Your fly speed is equal to your land speed, "as normal," but the bonuses don't double-dip if you gain a bonus to your speed from two items. If an item or spell increases your speed(not specified), it increases your land speed, and any bonuses to your land speed are considered when calculating your fly speed with the Fly speed, Soaring Rune, or similar effect.