You definitely will find variance in hash rate, and it could be more than 24 hours - because at some point you'll run into 24 hours of +10% hash and you'll be patting yourself on the back for your masterful tuning of the rig.
The nature of mining is that most PoW solves are probabilistic in nature, so your estimated hash rate will become more accurate the larger the sample time. For instance, your variance might be 10% for 1 minute, 5% for one day, and 1% for one month.Beyond the general affect of variance, you might also just confirm that the card temps are running as expected - so if they were running too hot and being throttled you'll be getting less.
Finally, when you're looking at multiple rigs, some pools wrap in stale shares and other hash-affecting things into your overall hash rate, so it will appear lower - although once again, if this number (like stale shares) is overly high, you probably need to look into things and will cause less than expected hash performance.
Hope that helps.