Edit2: I gotta have to agree with wims. It feel unnatural for me to use the '=', even though it might have been a problem earlier I haven't seen it at all. So it feels better to do it the way I've always done it. Sorry mate. Interesting read though.
Edit1: Looking back at the code I posted here, I see that I accidentally kept a ( bracket in the first %variable when I copy pasted from your code. If you try fixing that, can you try the script again JB? I get the feeling that it is what you want.
[23:46] <@nillens> !coins
[23:46] <@nillensbot> *Nillen has 58388 total coins.
[23:46] <@nillens> !watchtime
[23:46] <@nillensbot> *Nillen has watched the stream for 28wks 6days 17hrs 40mins.
using the same script, modified to use my own points system.
on *:text:!watchtime:#: {
var %points $coins(read,$nick)
var %time $calc(%points * 5 * 60)
var %duration $duration(%time)
msg # $nickpref($nick) has watched the stream for %duration $+ .
} 
