Depends on battery, wiki says:
Open-circuit (quiescent) at full charge: 2.10 V
Open-circuit at full discharge: 1.95 V
Loaded at full discharge: 1.75 V
Continuous-preservation (float) charging: 2.23 V for gelled electrolyte; 2.25 V for AGM (absorbed glass mat) and 2.32 V for flooded cells
All voltages are at 20 °C (68 °F), and must (for a 6 cell battery) be adjusted by ?0.0235 V/°C for temperature changes.
Float voltage recommendations vary among manufacturers.
Precise float voltage (±0.05 V) is critical to longevity; insufficient voltage (causes sulfation) is almost as detrimental as excessive voltage (causing corrosion and electrolyte loss)
Typical (daily) charging: 2.37–2.4 V (depending on temperature and manufacturer's recommendation)
Equalization charging (for flooded lead acids): 2.5 V for no more than 2 hours.[citation needed] Battery temperature must be absolutely monitored.
Gassing threshold: 2.4 V