The amount of recording time the UniFi NVR can store will always depend on camera bitrate. Multiply the bitrate by the time; then divide that number by the appropriate amount to get your size in MB or GB. See the example below, showing the calculation for one hour, with one camera stream, on full time recording.
(((3000kbps * 3600s) / 1,000^2 ) / 8) = 1.35GB
Or, for Gibibytes:
(((3000kbps * 3600s) / 1024^2 ) / 8) = 1.29GiB
NOTE: With motion recording, and adaptive bitrate (versions 3.9.2 and up) your results will vary.
If more space is needed, add an external disk to the NVR appliance.