How long does the effect of fillers usually last?

I’ve been looking into fillers because I’ve been self-conscious about areas like my lips and cheeks looking slightly lacking in volume. Since I’ve heard that this isn't a one-time treatment and requires repeat sessions, I’m most curious about how long the effects last. I’d like to know if the duration varies significantly depending on the area, and if the interval between treatments changes from the first to the second. I’m also curious how long the results you were satisfied with typically lasted. Getting treatments too frequently is a burden, so I would appreciate it if those of you who have had fillers could share your actual experiences regarding how often you schedule your treatments! I’m also curious about how painful it can be.