For normal posts there is 30 days limit. But that can be extended:
Option 1:
You can turn your post into “wiki post” so it can be edited indefinitely.
There is a catch - wiki posts are meant for shared database of community knowledge. They are editable by all users of trustlevel 1+. For safety reasons post editing history is kept so you can undo some changes.
Just use 
icon below the post and then “Make Wiki”.
Option 2:
If you reach status “Leader” you can edit anyone’s posts anytime.