I'd suggest that you take a few days and get to know your starter and its cycles. You might want to find some sort of container that you can mark - either with a pen or a piece of tape or you can tape a strip of paper vertically on the container and use that. Feed the starter and then just watch it.
Every hour mark the container as to the level of the starter.
Check it after 12 hours. If it's started to separate and form hooch, feed it again. If not, leave it for another 12 hours. If you starter is a very thin consistency, you may instead have a couple of inches of foam on the top and not so much bubbling within the starter. If your starter is thick enough, it will have at least doubled in volume.