OBS has no recording limit. I see no problem with 88 hours recording, as long as your hardware is working properly. I remember reports that after some hours the video of a webcam becomes black and the webcam has to be reinitialized to get a picture again. I assume this is a sign of bad hardware - improper cabling, bad webcam product quality. But OBS itself has no issue with long running recordings, no memory leaks or such. And no, webcams do not overheat if they record over a longer period of time.
If you record to ntfs media with flv or mkv format, you should be able to record without limit (ntfs) and without losing your video in case of a crash (flv or mkv).
The file size depends on the encoding settings. For recording, you will use a quality-based rate control method such as CRF or CQP. If the thing you're recording has mostly still images, there will be almost no growth of the file while nothing happens. The problem with this approach is that you cannot predict the file size, but it will probably be much smaller than with a bitrate-base rate control like CBR. With CBR, you waste space even for still images. With constant bitrate, you can compute the file size. 88 hours with bitrate 4000 (=4 mbit/s) would be 88 * 3600 s * 4 mbit/s = 1.267.200 mbit, which is 1.267.200 / 8 = 158.400 mbytes or 158 gigabytes. This is probably the upper limit for a CRF or CQP recording. A real CRF or CQP recording with mostly still images would probably 1/4 to 1/8 of that size.
You should definitely do a test run. Record your coffee machine for 4 days straight as if this is your experiment. And I mean it: record the whole 4 day long video. This will also give you an idea of the file size. Then act as if this is the real video of the experiment. You plan to postprocess it for sure, so try to do your postprocessing on the coffee machine video, so you get the feel how it is to load a 4 day long video into your video processing software. It's quite different to handle such a video than to deal with a few minutes or 1-2 hours long video.
If all runs well and you know how to deal with that video, do your experiment with that setup.
