While the time to process five birds is quite long, especially if done serially, I often can do eight or nine a day which boosts the tanning skill a lot. Washing and starting the first stage of tanning is done in less than an hour I then have plenty of time to cook the joints and the fat and do other chores. After the second stage which does take longer I have a couple of hours for other jobs and depending on the time of day I complete the process before sleeping or the next day.
Initially many leathers are of low quality, but within a few weeks I begin to get more decent or fine leathers. Usually I make clothes out of everything which also bumps skills up. I can see no reason not to process any skins.