The amount of data that a 1 hour youtube video uses up will vary depending on the quality of the video.
For a lower quality 1 hour youtube video you might only use a little under 1 GB for an hour long video but if it's higher quality you'll use at minimum 1 GB of data per an hour long youtube video and for some videos if they're of really high quality you may even use up to 3 GB of data for an hour long youtube video.
But expect to use at minimum of 1 GB of data for an hour long youtube video.