Gas companies usually read the meters on a monthly basis. We do however have plants that have telemetry for daily monitoring because the plant balances there gas transportation on a daily basis through the interstate pipeline - but this is unusual. As far as how often to read the meter yourself, I'd say it depends on what you want to accomplish. You should read them at whatever interval might provide you with useful insight. If you suspect a daily change - read them daily for awhile. If you find something interesting you can focus in by decreasing the time interval. I've read them at odd times related to the end of production cycles to see how much usage is occuring during non-production periods.
Whatever you do, particularly if this is a manual system, I'd encourage you to only read the meters if (and only for as long as) you will be using that data. People will be more conscientious about taking good readings if they know that something useful is being done with the data and that the task will go away when that information ceases to be useful.