How often should nails be replaced?

It's so uncomfortable when I do something on my nails that I haven't had a manicure or nail polish in a very long time.

But now I'm thinking of trying a nail polish manicure. Usually, how long does it last when you get a manicure? And I'm also curious about how often I should replace it.

I'm also curious about the approximate cost. Please let me know ^^;