There is no simple answer to this as it will vary wildly, but to give a ballpark figure, currently transactions average around 2kb, and with the current rate of transactions taken from moneroblocks.info we can see it's ~2 transactions per block. Given that the block transmission is every 2 mins, that means it grows ~2.8 megabytes per day simply from transactions based on current numbers. The current blockchain size is around 7.5Gb.