Generally speaking 755 is recommended.

Technically folders like the concrete folder don't need _any_ write permissions, only the following folders are actually written to by concrete5:
- updates
- packages
- application/files
- application/config

Depending on the way you set things up, sometimes permissions initially start completely without write permissions, and you add write permissions for the above folders. But you don't have to do it that way, really just mentioning that for completeness.