Theoretically, assuming material and material combinations of ball and seat taken care of ('best possible' for instance high-precision polished stainless steel in combination with suitable highgrade soft sealing FKM (viton), 'Nylon', 'Delrin' or others), life will depend on material wear.
1. Material wear will again depend om abrasion. Use fine-filter to protect as best as possible, both main intake for hose/pipeline and for pistol itself. Cleaning regularily not to take too much of input pressure. (Ref best types of for instance household washing machines.)
2. Other contribution is pressure peaks. Hence pressure peaks to be damped by constructional details. Contrary to popular belief this is done by checkvalves that is closing so fast that return pressure peaks do not have time to build up to max.
Example: larger ball checkvalve with 'sidetrack' for ball, gravitational return, no spring: among the slowest and high pressure peaks possible. Nozzle check valve with spring soclled 'soft closing': already closed when delta P is zero, low or diminutive pressure peaks, well within limits.
Conclusion: High-grade ball valve to be selected. (If this is your optimum. Other constructions for soft-closing could perhaps be evaluated, conferring with producers/suppliers and application.)
Check ballvalves should at least have a firm (wear resistive) guiding for the ball, straight and short travel to close. Spring assistance?
For soft materials (elastomers): high quality to optimum grade of aldering, UV- considerations if exposed, swelling and deformation, material disformation (floating) if exposed to excess pressures (pressure peaks or perhaps possible deformation by lower or uneven pressure by fast flowing leaks through tiny crack openings when else meant to be closed)
Price/lifetime considerations, ask if supplier can give guarantee or approximate lifetime depending on cycles?
