How often do welfare checks arrive in the US and how much are they?

I believe they're biweekly, though it's surprisingly difficult to find this information. They might be monthly.

As far as the amount goes, it depends on the exact circumstances, and it's really not possible to give a specific dollar amount since it varies depending on (among other things) the number of people in the family, if any of them are pregnant, if any of them are disabled, etc.