The problem that we had: we used a too fast horizontal rate and the scope had a limit of 32 buffers. After that, the data was overwritten. By using a horizontal rate of 1000s/division, we get about 100 samples/s, which is enough for our purpose. It doesn't even fill one buffer. A buffer is 10 divisions, and at 1000s/division, we have a total duration of 10000s (2.7h).
For info the 32 buffers is the default in the software, you can actually request up to 10000, although the number you actually get will depend upon the timebase, requested samples per buffer, and scopes memory (2Gb for the 6404D). The number of buffers is settable in tools->preferences->sampling