How much data you need can vary greatly depending on how much you use your phone and where you use it.
For instance, if you use your phone mostly at home for social, email, web surfing, entertainment and music, and other apps, you might have WiFi there, and may not need as much data. But if you use these services on the go where WiFi isn't available, you might want to consider more data for your mobile plan, and that can get costly.
Data is the most costly part of any mobile plan these days, outside of the obvious cost of the phone, and it can rack up based on use.
Take watching movies: if you plan on streaming from Netflix, Stan, ABC iView, Disney+, or any of the number of entertainment services available on your phone, standard quality video could consume up to 700MB per hour, while high definition (HD) could chew through between 1GB and 3GB per hour.
Music streaming services can consume data, as well, and depending on your audio quality settings, you could be munching through mobile data at a rate of between 40MB to 400MB per hour. Most people will likely hit under 100MB per hour, but the more you turn those quality settings up, the more your downloads are likely to feel it.
Web browsing also makes a stab for your data, and the more you do, the more downloads you're dealing with. There's no standard number for how many downloads you might go through, but if you use your phone a lot, you may want to expect between 50 and 250MB per day just for web browsing.
The point is this: anything and everything using the internet in some fashion on your phone requires a download of data in some form, and your download needs also have to cater for upload needs, as well. Uploads count alongside downloads in a mobile phone data amount.
So how do you work out what you need? Start with a guess, and then gradually increase the amount if you hit the limits a little too quickly.
Consider starting at 40 or 60GB, and if you need to increase on a monthly basis, you'll find phone companies are typically entirely happy to let you do so. Alternatively, you may find these are too big, and you can go down.
Many phone plans in Australia even capped monthly downloads at a slower speed, so when you hit your 40 or 60GB monthly, the plan is capped, and the rest of the downloads are unlimited at a slower speed. If you opt for one of these plans, the data amount you pay for – that 40 or 60GB – will be downloaded at the fastest speed available to your phone and the network at the time. Once you hit the maximum monthly download limit, the speeds will drop to something much slower, but still provide internet access.