In general, the longer the scale length - the more stable a string will be as the tension is lowered and the pitch follows.
A thicker gauge string will allow you to fudge the numbers a bit and get lower, but usually at the expense of intonation, which in terms of what you're looking for; you'll need to adjust for anyway.
Anything is possible with some mild modifications (like very large lower strings to get way low) but depending on the style you're going for and how you play, you can get some pretty good results with the standard 24-25" scale length guitar to about drop C (CGCFAD) without needing to change all that much.
I'm currently using a project SG with .11's and even though it's the shorter Gibson scale length (24.75"), it can djent down to drop A pretty well, even with what's considered "light" gauges for that tuning. (AEADF#B).
There are a few factors at play when it comes to tuning down on the guitar.
- String Gauge
- Scale Length
- String Tension (Really, a combination of gauge + scale length + desired pitch)
As long as you keep these in a well-balanced ratio, you can tune down to your heart's desire (although, eventually you're guitar amp isn't going to like the low frequencies). The lower you go in tension/pitch, the bigger you'll need either string gauge and/or scale length to be. One factor is exponentially harder to adjust than the other.
Also, an intonation adjustment will more than likely be necessary after any of these suggested changes.