question for everyone with bags and NO compresor

for all of you that have bags and run tanks with no compressors, how long does a nitrogen bottle last? I just picked up for free a large air tank and by large i mean 4 feet by 19 inches or so. I was wondering if i could fill it with either air or nitrogen and go without a compressor and just keep watch on my tank so i know when to refill it. does this sound reasonable?