Does playing strokes which apply more force to the strings, like double stops, spiccato and sautille, wear out the strings more quickly?
2 Answers
No. Strings tend to wear out as much by losing elasticity as by having their surface wear down.
But in any case, you should not be applying more force when using the bowings you mention. Yes, there's more force when doing a double-stop, but it's spread over two strings. Make sure you aren't "grinding" when playing multiple stops.
The various bounce and throw bowings (including spic. and saut.) should never involve direct pressure application. You should be dropping the bow fast, but holding in lightly enough that it rebounds on its own.
About the only time extra force is needed that I can think of (other than fffff passages) is when you want to "snap-start" by fully engaging the string with the bow hair, and starting the note with a release of downward pressure as your stroke initiates.
Change strings when you feel them not as bright as before. I also find the question strange. You shouldn't be afraid of your strings wear, practice when you want to or need to. Change them when you need to.