Shockingly little — this is one place the sales guys undersell. Panels have no moving parts and run 25-30+ years. The honest maintenance list: watch your monitoring app (5 minutes a month), clean the panels once a year in low-rain areas like ours (before summer, after pollen season — de-ionized water and regular soap, never when panels are hot, or just hire a cleaner), and a system health check every 5-10 years.
The components that can actually need service over decades: inverters (that's why warranty length matters — 12.5 vs 25 years is a real difference) and batteries (10-15 year warranties). The panels themselves? They'll likely outlive your roof.