Want to solve a real world math problem about Pokemon Go? Here you go!

A friend of mine recently brought up the following problem. As an avid Pokemon Go player he is aspiring to collect all 26 available types of a particular Pokemon. So each time the Pokemon called 'Unown' spawns it presents itself in one of 26 different forms. Gotta Catch 'Em All?

So the question is how many "Unown's" on average one has to catch in order to get all types? So let's say you catch one 'Unown' Pokemon each day - what is the expected value in days to catch 'em all?

How do you approach this problem? Can you help my friend to get an idea how long he has to keep on playing?