How low you can go comes down to strings and scale length. The further you tune down, the looser the strings feel, until notes buzz, go out of tune when you play hard, and lose clarity.
A rough guide for a standard-scale guitar:
- Drop D to drop C: regular sets work; many players move up one gauge.
- Drop B and B standard: .012 to .013 sets.
- Drop A and A standard: .013 or heavier, and it's about as low as most standard-scale guitars stay playable.
Going lower:
- Baritone guitar: a longer scale keeps low tunings firm and clear. See baritone guitar strings.
- 7- or 8-string guitar: adds lower strings instead of retuning the six you have. See 7- and 8-string guitar strings.
Any big drop in tuning is worth a setup afterward. See setup after a lower tuning.