I have read that DSLRs have a max video length of 30 min for various reasons, but do cell phones have a similar max video recording time? If I buy a 64 gb microSD card and record HD, will it cut off at an hour or at the battery/memory full markers?
Some do, some don't. It depends on manufacturer but the ones I've seen will record until it runs out of memory. I believe the older android OSes had a limit of 2GB per file and newer ones have a 4GB limit since Android uses FAT32 as the file system.
I'm looking to get the new Sony Z2 or Samsung S5 for the 4k capability and am wondering if I'm going to run into any issues recording videos of longer length. I would figure they would solve those issues by now with video that advanced but I am having my doubts.
Unless newer versions of Android use an alternate file system, all of them will have a 4GB file limit because of FAT32. So your recording time will be limited only by your video recording quality. Recording at 4K resolution would eat up a 4GB file pretty quickly versus at 720p. As I said in the first reply, some may have a software limit on how long you can record any quality regardless of how much space you have. If you want unlimited recording time, you need to use a digital video camera instead of a photo camera or a cell phone.