How much squinting can damage your vision over time?

None. Squinting cuts out some of the light that is not properly focused on the retina. It might cause wrinkles, but it won't hurt your vision. Your vision may degrade over time due to age, not due to squinting.
Very little. This will not have much if any effect on the vision. It may however cause the development of wrinkles on the skin involved in the squinting.