Hi rekloos-
Glad to help out here.
Firstoff let's consider speed:
You can record at whatever speed you like, but 7" reels usually have recordings at 3.75ips or 7.5ips. 10" reels are normally recorded at 7.5ips or 15ips, but again the speed is entirely up to you.
Now lets consider tape thickness and tape length:
Two variations are quite common: 1- The "standard" tape thickness is 1.5mil. This is your "toughest" tape in terms of resistance to breakage and print-through. 1.5mil tape is wound on 7" reels in lengths of 1200' or 1250', depending on the manufacturer, and wound on 10" reels in lengths of 2400' or 2500', depending on the manufacturer. 2- The "long-play" tape thickness is 1.0mil. A 7" reel will spool 1800' of 1mil tape and a 10" reel will spool 3600'. These two lengths are almost always what long-play tape is available in. There are also "double play" and "triple-play" tapes, which aren't common and aren't even being made anymore, to my knowledge. You really need a fine machine to safely handle these last two tape thicknesses because they are delicate and prone to stretching and breakage, so for our purpose we will only consider "standard" and "long-play" tapes.
Now consider recording time:
This is pretty easy to figure. For example, a 10" reel with 2500' of tape at 7.5ips: 2500' is 30,000in. And 30,000in/7.5ips = 4,000sec. And 4,000sec = 66.66 minutes. As you can see, what we did was convert feet to inches to get total number of inches. Then we divided speed (in this case 7.5) into total inches to get total seconds of recording time. Lastly we divided total seconds (4,000) by 60 to get recording time in minutes.
Here is a quick summary for you. I saw a really good table just the other day, but I'll need to look for it and get back to you on that:
7" reel
1200' @3.75ips = 64min. @7.5ips = 32min. @15ips = 16min.
1800' @3.75ips = 96min. @7.5ips = 48min. @15ips = 24min.
10" reel
2500' @3.75ips = 2hr, 13min, 20sec. @7.5ips = 66min, 40sec. @15ips = 33min, 20sec.
3600' @3.75ips = 3hr, 12min, 00sec. @7.5ips = 96min, 00sec. @15ips = 48min, 00sec.
I did have to work this out on the calculator, but after you live with these numbers for awhile and end up using one or two speeds and a couple of different tape lengths most of the time, you won't even have to think twice about this.
Cheers, and happy reeling!