Well in the Nautilus X I'm vaping almost exclusively Red Astaire flavour (which I don't think is overly sweet?), 6mg, 50/50, 1.8 coil at between 14.3 and 15.7 watts (it's rated 12-16 watts) - as said before, am priming and leaving for about half hour before using. I'd say an average hit for me is about 3s. And I probably get through 2ml and 4ml of juice a day. All MTL.
From what's been said, sounds like the problem is either:
1) starting off at too high a wattage - I usually start at around 14 with a new coil but within a few hits will move up to the range I mentioned above - maybe it's worth me vaping a new coil at around 12watts (or even lower?) for a while before moving up to the more flavoursome range?
2) vaping like smoking, i.e. too many hits in a row, until not enough juice in cotton and it starts to get burnt.
Things is, 1) Is easy enough to remedy, but 2) less easy... Sometimes you just want a good few hits in a row, and often I think I do this without even realising e.g. while concentrating on something else - and from what I understand, chain vaping till cotton too dry even once can be enough to ruin a coil.
I have read the Innokin Zenith or Jacvapour coils last longer, so may get one of these, or try the whole RTA thing (have another thread on the go about this btw, sorry for any overlap!)
Anyway, if there's nothing else it can be, at least I know what's going on and can try and avoid.
Though is it possible to be more specific on how many hits in a row is too much (e.g. 3s hits), and how long to leave in between to be safe?