According to the levels I see in the Lemmings Archive, it looks like everyone only makes small levels. I'll remake the small levels of mine. I feel rewarded when I beat my hard and long levels, since I have a lot of free time to play long games.
Don't feel like you have to make small levels. What you see in Lemmigs Archive pretty much just means most of us are lazy.
I mean really, it's just the physics of time--larger levels take longer to make and to playtest, so there is naturally bound to be a higher number of smaller levels, especially when the incentive is mainly fun as opposed to profit.
I might actually have some free time tonight (gasp!) so, that would be the perfect time for me to try your long level. I do have to say, from the first look of it I can actually see why you might've started off with a time limit of 15 minutes. Not sure whether I'd consider it a good thing or not--we'll see.