@Simon4s you could use 12GB on first day or on last day of your billing cycle
They just call it a 4GB "average" per month but you are in no way restricted how you use it
@Simon4s It hard not to get confuse but remember the $40 plan is ACTUALLY $120 every 90 days.
@Simon4s I believe your question has been quite adequately answered by @Rockdaddy22, @kav2001c, and @Luddite, but please let us know if you need any more clarification on anything. Also, we weren't quite sure what you were talking about in regards to have 5 or more gigs remaining during any month.... but rest assured you have *twelve* gigabytes of data to use however you see fit over the course of 90 days, with no per-month limit or set amount. In fact, "months" do not enter into the equation at all (90 days is never exactly 3 calendar months, for example).
This 90-day arrangement gives me a lot more flexibility. My previous Telus plan was $55/5GB (grandfathered plan from 3 years ago). It's a great plan compared to the current offerings of the "main brands", but there was an incident several months back when for one month when I was under my 5GB limit, but the month after I went massively over and ended up paying extra $$$ for data overage with no ability to prepay any add-ons in advance.
Not sure why this is very difficult to understand, though the marketing material does not help. The marketing meterial for the 90 day plan breaks it up into "per month" chunks, I suppose so people can more easily compare against other monthly plans elsewhere. However you have to think of a 90 day plan exactly as that. It's the same idea as a monthly plan, but for a period of 90 days.
So you get X amount of data per 90 days, you can use that data at any point during that 90 days before it resets to the next 90 day period. If you use 8GB of a 90 day 12GB plan in the first 30 days you've got 4GB left to ration out for the next 60 days after which it will reset back to 12GB on the 91st day..