Chlorine, salt and sunscreen all attack elastane, and they keep working until they are rinsed out.
The short answer
every wear
When to wash sooner
Wash it now if
- it has been in water
- it has been worn in the sun
What to do between washes
Rinse in cold fresh water immediately, before it dries. That single step doubles the life of most swimwear.
How to wash them
Hand wash cold, no wringing, dry flat in shade. Never tumble dry and never leave it rolled in a wet towel.