As Steve said - the normal data on the SD card really doesn't take up all that much room - unless something drastically changes/has changed over the x36 models.
Recordings can take up alot over time (depending upon how busy the system are that you are monitoring) but on the x36 models, the radio/firmware results in a crash/reboot (disabling recording) once you get to/over 2Gb of recordings.... this then disables future recording without manually intervention by the user. But that's the x36 models....
The SDS-100 appears to have improved RECording such that (at least) the RECording setting is now "sticky" - that is, the radio remembers that you enabled recording when the radio is power cycled. I'm running full time recording on my loaner SDS-100 to see if the radio still crashes like the x36... I'm currently at just over 1 Gb (17,000 recordings) and the card is still reporting 6.2 Gb available....