The common rule of thumb is that a lead acid battery should not be discharged below 50% of capacity, or ideally not beyond 70% of capacity. This is because lead acid batteries age / wear out faster if you deep discharge them. The most important lesson here is this:
Since that is no longer an issue (and never was an issue with lead acid batteries) there is not a need to fully discharge. By discharging a lead acid battery to below the manufacturer’s stated end of life discharge voltage you are allowing the polarity of some of the weaker cells to become reversed.
Sealed lead-acid batteries are generally rated with a 20-hour discharge rate. That is the current that the battery can provide in 20 hours discharged to a final voltage of 1.75 volts per second at a temperature of 25 degrees Celsius.
The ideal discharge curve of a lead acid battery is on a flat discharge curve, the amount of current that the battery can deliver remain more or less constant for quite a while and then drop off rapidly when the limit of it capacity has been reach.
It turns out that the usable capacity of a lead acid battery depends on the applied load. Therefore, the stated capacity is actually the capacity at a certain load that would deplete the battery in 20 hours. This is concept of the C-rate. 1C is the theoretical one hour discharge rate based on the capacity.
Lead acid batteries should never stay discharged for a long time, ideally not longer than a day. It's best to immediately charge a lead acid battery after a (partial) discharge to keep them from quickly deteriorating.