Using dongles for internet access can work out very expensive. I can't find the specific dongle offer you refer to but this link (selecting PAYG and 4G)
http://www.vodafone.co.uk/shop/mobile-broadband/dongles-and-mobile-wi-fi/index.htm#device_2
suggests that £50 gets you 30Gb of data (after you've paid £45 to buy the dongle in the first place). Watching Youtube for an hour uses up about 0.43Gb, so if you only watched Youtube videos you'd get about 70 hours worth of access spread over 90 days, which works out about three quarters of an hour per day. Alternatively you could use your entire 90 day allowance to download just 7 movies in HD.
So if you're planning on doing anything which involves streaming audio or (particularly) video services, or downloading large files, you might well find that you've got a data allowance which is nowhere near big enough. However if you simply do some general web browsing (such as internet shopping) and sending emails you should be OK.
The following examples of data usage are taken from the BT website. (Remember that they're 'per month' though. You'll need to multiply the figures by three for a 90 day dongle allowance):
10GB data:
Daily: Browse the internet for half an hour
Weekly: Watch two and half hours of online TV using a catch-up (like iPlayer) or streaming video service (like YouTube), play online games for one hour
Monthly: Stream one standard definition film, listen to 100 music tracks or 10 albums, upload 100 photos to Facebook or Instagram
20GB data:
Daily: Browse the internet for one hour
Weekly: Watch five hours of online TV using a catch-up (like iPlayer) or streaming video service (like YouTube), play online games for two hours
Monthly: Stream two standard definition films, listen to 150 music tracks or 15 albums, upload 200 photos to Facebook or Instagram
40GB data:
Daily: Browse the internet for three hours a day
Weekly: Watch eight hours of online TV using a catch-up (like iPlayer) or streaming video service (like YouTube), play online games for five hours
Monthly: Stream four standard definition films, listen to 200 music tracks or 20 albums, upload 400 photos to Facebook or Instagram