I want to apply a magic effect with a variable length, e.g. anything from 1 to 100 seconds, to a character. I could make a magic effect with the length of 1 second, 2, 3, ..., 100, but that's very cumbersome approach.

I've been thinking to make instead just a 1, 5, 10, 20, and 50 seconds long magic effect and then use a Papyrus command like 'RemoteCast' to cast one 10 second magic effect and two 1 second magic effects on the character to simulate a 12 second magic effect. Is that possible?

Don't know for sure, but I don't think you can cast spells to run after each other unless you cast them when you want to apply them.

You'll probably have to use a SKSE SetDuration function to achieve the results dynamically.

If you don't want to use SKSE (why not?), you'll probably have to create those 100 effects and spells.

Hm, the command 'SetDuration' is not in the list of Papyrus commands in the wiki of the CS:

http://www.creationkit.com/Category:Papyrus

Is it a command that was introduced in the new 1.70 alpha version?

You're looking for http://www.creationkit.com/SetNthEffectDuration_-_Spell - which was already part of last SKSE build, so 1.7 isn't necessary.