Actually, chickens only need grit if they don't have access to the ground/dirt every day. If they are in a chicken run that is on the ground or free range they will get the grit they need from small pebbles and/or sand, etc.
If they don't have access to dirt then they do need access to grit 24/7.
If you aren't sure that they are getting what they need then you can always leave some out for them and see if they use it.