Still working out my wonky issues, but personally I'd only turn mine off if I knew I was not going to use it for a significant period of time, say more than 2 weeks. 2% per day battery discharge in sleep sounds rather high considering the expected battery life on a full charge of 60 days at half an hour or an hour depending on the source you read, that would be 120% battery discharge in those 60 days if it was asleep between that half an hour or hour of reading, which I suspect it is for those measurements.
Sleep power use should be in the realm of 1-2mw, which works out to about 1% per day if 2mw and .5% per day in sleep mode for 1mw (1.1mw is approximately the minimum amount required to keep from losing memory state from LPDDR1, probably varies per LPDDR1 manufacturer though, but I doubt it is far off from that). Shouldn't need much else in sleep mode other than keeping memory state.
Battery self discharge should be roughly .5% per day on its own however for most/all rechargable lithium ion batteries (discharge/overcharge protection circuit used on smart LiIon batteries uses a small amount of power all the time and is independent of the device hardware).
So should see an average 1-1.5% day in sleep mode. Taking those 60 days in to account, I am thinking you should see about 1% in sleep mode and about .5% while off, at least if you stretched it out over a long testing span, like a full month or even 2 months.
No matter the use case, the battery should last about 4-6 years before you start noticing any appreciable drop in battery life. You just don't go through all that many charge cycles on something with roughly a 1 month battery life. Now a tab that is used a lot and burns through its battery and has to be charged up almost from empty every day, that'll probably show appreciable loss of battery capacity after as little as a year or a year and a half, probably ~2 years. LiIon batteries tend to withstand about 500 charge cycles to drop to about 75% new battery capacity. Of course the battery capacity will start going faster after that, so at 1,000 cycles you probably have less than half the original life left.
About 7 years of shelf life without significant use to get to 50% new battery capacity.
Personally I don't think I plan on getting rid of my nook ST, even if a nice shiny new toy comes out. At least not for awhile. Some nice new eink reader comes out that is worth while upgrading, I'll do that. Then my nook ST is going to live on as a backup and/or hazardous use ereader (IE for when backpacking instead of taking the nicer one, though right now the nook ST is going to be going backpacking the next time I go). All said, I don't expect to get even 4 years of life out of it. I'd expect there will be something new and shiny (or matte, I prefer it in matte) to get me to purchase it in the next 9-18 months. After that, probably another new upgrade within 2 years of that and so on. At least for the foreseable future.
Ereaders seem to be a little slower in "new models" than tablets and phones are, but there still seems to be a roughly 18-24 month release cycle. Unless the next models are barely incremental, for $99 roughly (so long as prices don't rise that is) I don't mind getting a nicer one when they come out.