Generally about the same as a kinetic, I think the early Solar Seiko watches average about 8 years.
I had one of the Solar Chronographs which ate two capacitors in as many years though, given that Seiko have only fairly recently jumped back into Solar watches they aren't as reliable as Citizen, who do virtually nothing but eco-drive.
Changing the cell is very easy though, they cost £10-£15 each and it's just like changing a battery. Kinetic watches are a bit more fiddly and need an expert or a competent "tinkerer".