Is it more affective to work out everyday with shorter workouts like 8-15 min long ? Or to work out like every second or third day but with workouts which are longer like 20-40min ?
8 - 15 min isn't very long, but is enough time to run 1-2 miles which is much better than doing nothing. If possible it'd probably be best if you could combine the two, on the days you can do a full body workout for 30 - 40min (at at quick pace) and the days your lacking on time just try and sneak in a 10 min run. With at decent diet that's enough to see solid results.
You should try 5krun which is much better for health. I do three hours on the track & gym you should try it brilliant.