now the video time limit can be disabled - thanks to nafraf!
the new limit for videos is only the FAT32 filesize limit (4GB) or the battery power, of course. 4GB is 20:22 min in 1080p and 25:13 min in 720p with cameras default quality settings.
i tried to reduce the bitrate with "video mode" set to "quality" and lower values than 84. with quality set to 42 a 720p video filesize is only about 820mb with a length of 30 min, for example.
the new option is in the "Video Parameters" called "Video without Timelimit". it is disabled every time the camera starts because of the possibility of cmos overheating, so you´ll have to enable it every time. the max cmos temperature during my tests was 43°C after half an hour of HD video, so i don´t think this is really a problem.
again: thanks to nafraf!
the CHDK version below is for elph100hs/ixus115hs with firmware 101a.