I'm slowly being drawn to GG, just wish the "gigabags" lasted more than 30 days !! I can see me using the setup at home, simply to burn off unused Gb's ....not the most efficient way of operating, as I have unlimited BB at home anyway !!!
You'll struggle to find a provider who doesn't limit the time a data bundle is valid for now, other than using pure PAYG (which is always expensive if you use a fair amount). O2 do a starter pack valid for 3 months, BUT subsequent topups are only for a month.
I'm never sure how much data I will use - most of the time, none, as unless away for weekend/week etc, I just use my phone out and about (which has (currently) unlimited data (but not for tethering)), so the laptop stays at home.
If I am going away, and taking the laptop, I buy the £5 Gigabag for my data SIM, which goes in my dongle/MiFi. Thats 500Mb, which is more than enough for me for a week (I bought a Cypriot 500Mb SIM for Cyprus last year, used about 150Mb).
But you can never be sure how much you need - you may have to go over that amount (and at 2p per Mb once you go over the bundle amount, it can add up if you way over). So my little workaround is:
I have a 2nd GiffGaff SIM, purely for data use, but without a Gigabag on it. If I'm away, and the data allowance on the 1st Gigabag is getting tight, I quickly buy a Gigabag for my 2nd data SIM, and swap the SIMs over. The size of gigabag needed for this 2nd SIM I estimate based on how far into my holiday I am.
Obviously, if you do consider getting a 2nd data SIM, wait until your first arrives, then refer yourself, so you get the bonus

Note, to stop SIMs going inactive, I send a text message (from my main mobile, which has unlimited texts) to all my other SIMs, and periodically put those SIMs in a device to retrive text. Not sure if you have to do the latter, but I do for a belts-and-braces.