What is the number of days that can be recorded on a 1 GB SD memory card when using 4 ch at a 30 second interval with SMARTDAC+ GX/GP series?

Approximately 5400 days. A quick calculation with 12 bytes/ch (display data) and time data 16 bytes/sample is as follows:

12 × 4 ＋ 16 = 64 bytes
1000 M/64 = 15.6 M times
15.6 M × 30 sec
Approx. 5400 days