For that hull, exact spacing is all that critical since the hull is relatively light and the hull is solid fiberglass.Seanic4256's spacing is probably a good number.
If the hull is out of the water, on a trailer, you can check the spacing by hitting the hull across the with with a plastic faced or dead blow hammer and listen for changes in the thuds. You may not be bale to hear the changes at the stringers exactly but you should be able to tell where the stringers aren't. The thuds will be hollower sounding. Find several hollow areas along the hull and the stringers will be in the middle of those lines.
It may also be possible to see/feel and measure the stringer offsets from the keel from inside the bilge, aft or at the tank area.by probng through the deck plates. There is a good chance that stringers are on either side of the tank.