Earth Glide (Ex) When the creature burrows, it can pass through stone, dirt, or almost any other sort of earth except metal as easily as a fish swims through water. If protected against fire damage, it can even glide through lava. Its burrowing leaves behind no tunnel or hole, nor does it create any ripple or other sign of its presence. A move earth spell cast on an area containing the burrowing creature flings it back 30 feet, stunning it for 1 round unless it succeeds on a DC 15 Fortitude save.
I believe you default to the creature's burrow speed. If the creature is using a spell or spell-like ability that grants them Earth Glide hopefully there are some additional details describing the proper speed /adjustment to use within said ability/spell.
Yar.
I assume you are talking about the Eathglide Revelation from the Stone Oracle?
If so:
You can pass through stone, dirt, or almost any other sort of earth except worked stone and metal as easily as a fish swims through water. If protected against fire damage, you can even glide through lava. You glide at your base land speed. While gliding, you breathe stone as if it were air (you do not need to hold your breath). Your burrowing leaves behind no tunnel or hole, nor does it create any ripple or sign of your presence. A move earth spell cast on an area where you are flings you back 30 feet, stunning you for 1 round unless you succeed on a DC 15 Fortitude save. Activating this ability is a free action. You can glide for 1 minute per day per oracle level. This duration does not need to be consecutive, but it must be spent in 1-minute increments. You must be at least 7th level to select this revelation. You can bring other creatures with you when you glide, but each passenger costs an additional minute per minute of travel.
Bolded the relevant text.
For most medium sized creatures, that is 30'.
For most small creatures, that is 20 feet.
If you mean the monster ability (ie: Earth Elementals), Herbo has the right of it: you use their Burrow speed.
~P