You might be able to double the battery life by reducing the LED current, both day and night. 25mA is probably the maximum current for the LED, but you may not actually need anywhere near that much, particularly if there are 30 of them. And of course you could put two to four 18650's in parallel.
But I don't think running 30 LEDs for months on one 18650 is in the cards unless you use solar power to both power the LEDs and recharge the battery during the day, and then run the LEDs at very low current at night.
Are you using white LEDs? Vf for red might be around 1.8V at 10mA or lower, which might let you use two in series. That's right on the edge though, and might not work.
So I think you need to go back and re-examine what you really need - how many LEDs, how much current. Do they need to be on continuously, or could they be blinking, or perhaps fire sequentially? You might want to describe what you're doing in a bit more detail. Maybe someone here will have an idea.