The simple answer is: The redder they are. However, years ago, one of my neighbors let his tomatoes stay on the plant for a long time. They actually started to tear at the skin much like bananas do if you let them stay too long on the plant, and some shriveled up like raisins. I thought "What a waste!"
A tomato will ripen off the plant like a banana will, but it will take some effort on your part to help them ripen properly.
This is the first tomato I'm harvesting in a long time. I noticed what seemed to be a seam running along the side of it, and thought the skin might start ripping soon, so I harvested it.
Still green, but it will ripen with time.
Some people might choose to let their tomatoes stay on to ripen, but again this might cause the skin to break or tear. When this happens, you are exposing your fruit to unwanted insects, so it's best to harvest before that happens.
Others might suggest bagging fruit still on the vine/tree, but that seems like too much work for me.
Now, to help your tomatoes ripen quicker, you can stick them in a brown paper bag with an apple or banana (why not try both!), which give off a gas called ethylene. This gas quickens the ripening process in other fruits around it. This is what I do recommend and plan to do it with my tomatoes.
Note: Don't pick the tomatoes green if they aren't ripping, leave them on until red if possible.
If you grow tomatoes, do you harvest them around this time or wait longer?
Thanks Steemians!
@vegansilverstack