Any time you're ready, your leo will be ready for a larger enclosure.

Make sure it's big enough for 3 hides: warm dry, warm humid, & cool dry. I suggest a 20 long right now. That's 76 x 32 x 32 cm tall. I recommend a heat mat that covers 1/2 the footprint.
If your room temps dip lower than 67ish*F (19.4*C) you may wish to warm the air with a ceramic heat emitter bulb that's plugged into a separate thermostat.
Temperatures - A temperature gradient from warm to cool maintains your leo's health.
Here's a temperature guide for all leopard geckos as measured with the probe of a digital thermometer or a temp gun (and controlled by a thermostat set at 91*F/32.8*C):
- Warm end ground temperature: 88-92 F (31.1-33.3 C) underneath a leo's warm dry hide
- Cool end ground temperature: 70ish-75 F (21.1-23.9 C) Usually the cool end ground temperature matches the room temperature where the enclosure sits.
- no greater than 82ish F (27.8ish C) air temperature - 4 inches above ground on the warm end
- no greater than 75 F (23.9 C) air temperature - 4 inches above ground on the cool end
Leave the heat mat/UTH on 24/7. If you wish, during the night turn off overhead lighting/heating (~12 hours on and ~12 hours off) unless ambient room temperatures drop lower than 67ish*F (19.4*C).