Depends on how dirty the environment the tractor is operating in. Most of the manuals call for daily maintenance on the air cleaner - most of them are probably lucky if they get looked at once per year.
Probably changing the oil in the air cleaner when you change the engine oil is sufficient (and better than average) for the typical light-duty antique tractor at this point. If you're using it in dusty conditions, you probably want to increase the frequency. Generally, the manuals call for using the same oil in the air cleaner as you're using in the engine - 10W for very cold temperatures, SAE20 for in between, and SAE30 for summer. Idea is to have the oil fluid enough to do a good job of cleaning, but heavy enough not to get swept into the engine - and this is temperature dependent. Again, how closely you follow these guidelines is your personal decision, but keeping grit out of the air going to the engine obviously is pretty important if you want your engine to last.
