There are two restrictions in place.
Playback: Only 8180, 8-bit samples per second are supported by WAVSETA.
Recording: At 8180, 8-bit samples per second, MICSTART can record up to 30 (or maybe it's 32) seconds maximum unless you write some fairly complicated code to wrap around it. Normally the mic data is stored from beginning to end of the mic chip's internal buffer. (cont. below)
However if length 0-seconds is specified, it uses a circlular buffer, meaning that it wraps around and new data overwrites the oldest in the buffer. If you use this setting, you would need to continuously pull the data out of its buffer and store it in your own buffer, while it's still recording.
