Agree with FFF, 6 min marks work well as a starter - in fact it does not really matter WHAT timing marks you use as long as they are at a sensible interval and all the same, 6 mins based on calculated groundspeed is just convenient and fills the bill, otherwise just use a "thumb" and whatever it works out at is your time marker.
(personally I just draw on a track line and "thumb" distance for whatever I am doing - but that is NOT what I would teach a student).