I've had about 3 batches at different stages in my incubator before. It's a cheap incubator and you have to take the whole top half off to get into it but it's done good service. (I just lift it up from one side to take out and replace what I need to so as little heat and humidity escapes as possible).
If you have two batches close together I've not found the 'younger' batch is affected by a few extra days of higher humidity, and I like my humidity up at least 80%. And turning them is only super essential in that first week of incubation, so not being turned towards the end won't affect them. If you have two batches close together I'd wait an extra day to put the first lot into lockdown perhaps, but other than that I wouldn't worry.
To me there's no maximum amount of time between adding batches, you just have to adjust what you are doing to try and get the best result such as having them in a container to whip batches out for turning, or if the last batch you add has had a bit more higher humidity waiting for two other batches to hatch then they get a few days at a slightly lower humidity.
I've candled Button quail eggs every day (because it's fascinating) and still had all the viable eggs hatch. I had one egg that started to develop but then the embryo stopped growing, yet it was still moving around. It took about a week to stop moving (poor little thing) and never grew any bigger, but that baby was active. It was bizarre.
Heck, I'll candle to check if they have internally pipped because I just can't help myself! I don't think I should ever hatch duck eggs. The suspense would do me in!
