With a 20GB Cap your effective bandwidth is 0.064Mbps or 65Kbps. Which is just slightly above 56K ratings of modems.
1Mbps=0.125MB/s (8 Mega*bits* per Mega*byte*) =0.000122 GB/second =0.4395 GB/Hour =10.547 GB/Day =316 GB/Month (30 day month)
So for each Mbps you should be able to download 316GB/Month in theory.
If you invert this you get a factor of 0.0032:
So for every GB of bandwidth they give you, your effective bandwidth is 0.0032Mbps.
Bell gives you 20GB (from last I looked), so 20*0.0032=0.064Mbps /1024 means: you have an effective bandwidth of 65kbps
Welcome back to the dial up days folks! What you could download in the early 90's is essentially the same as what you can download today.
So remember to find your effective bandwidth, multiply the number of GB they give you by 0.0032
(To find how many GB you should have multiply your Mbps by 316)
CAP EFFECTIVE MONTHLY BANDWIDTH (30 DAYS) 2GB 6.4Kpps (typical Cellular plan) 60GB 129Kbps (fairly common cap, especially with Cable) 120GB 384Kbps (Higher End caps with many providers) 250GB 0.8Mbps (Tech Savvy's low cap, considered quite generous) SPEED THEORETICAL MONTHLY CAP (30 DAYS) 56Kbps 31GB (Dial-up Speeds) 512Kbps 158GB (Introductory Speeds) 1Mbps 316GB (Often categorized as the minimum speed to be called "high speed") 5Mbps 1580GB (Planned CRTC minimum available download speed, Typical DSL speed) 10Mbps 3TB (High End DSL speed, Readily available Cable Speed) 16Mbps 5TB (Premium Cable speed) 100Mbps 32TB(Older LAN speed, some countries offer this with fiber optic) 1Gbps 316TB (Gigabit, typical LAN speed these days)
TL;DR - With the default cap essentially you can't download anymore than you could've (in theory) 15-20 years ago. (Of course then you'd have to have your phone line dialed in all the time, so there has been some improvement)
edit: Here's a fun new calculation, at 5Mbps, you are paying $4.40 an hour (at full speed) for your overages (based on $2/GB Overage)