Salt, bird droppings, and the marine layer are the three things that wreck gelcoat in a slip. Salt crystals hold moisture against the surface. Droppings etch gelcoat within days in summer sun. The morning marine layer rewets everything so nothing ever quite dries.
A boat that lives in Alamitos Bay and goes out most weekends needs a wash every two to four weeks and a wax twice a year, spring and early fall. A boat that rarely leaves the slip needs the same wash schedule; sitting still does not keep it clean.
The sign you have waited too long is chalk on your fingertip when you rub the hull. That is oxidation, and wax will not fix it. It needs a compound first, which is why we do a test spot before quoting the whole hull.
A monthly wash is the cheapest way to never get there.
Related: Monthly Wash and Upkeep, Oxidation Removal. Or see boat washing and upkeep in Long Beach.