In start, I read few documents which clearly stated the maximum allowable charging rate as function of 20 hr capacity to be amp-hr rating/10. This still is valid for UPS which charges the battery on flat or same charging rate.
Your question is a good question, which prompted me to look at different sites. Currently following sites quotes this.
https://www.cdtechno.com/pdf/ref/41_2128_0212.pdf
[TABLE]
<tbody>[TR]
[TD]Current Limit as a function of 20 hr. rated Capacity
[/TD]
[TD]Approximate Max. rate Discharge period
[/TD]
[TD]% Depth of Discharge as function of the 20 hr. rated Capacity
[/TD]
[/TR]
[TR]
[TD]Amp-hr rating/1
[/TD]
[TD]15 min
[/TD]
[TD]45%
[/TD]
[/TR]
[TR]
[TD]Amp-hr/2
[/TD]
[TD]30 min
[/TD]
[TD]55%
[/TD]
[/TR]
[TR]
[TD]Amp-hr/3
[/TD]
[TD]1 hr
[/TD]
[TD]60%
[/TD]
[/TR]
[TR]
[TD]Amp-hr/4
[/TD]
[TD]3 hr
[/TD]
[TD]75%
[/TD]
[/TR]
[TR]
[TD]Amp-hr/5
[/TD]
[TD]8 hr
[/TD]
[TD]90%
[/TD]
[/TR]
</tbody>[/TABLE]
Note that above table is valid for deep cycle batteries but here in Pakistan non of the manufacturers claims their batteries as deep cycle. These ordinary lead acid batteries can only be discharged to 80% of their rated capacity before it requires to be charged again. So only content in above table valid for your case is the last entry.
Table only says, that if your lead acid battery is heavily discharged then you can charge it at higher rate initially but later charging rate has to slow down to Amp-hr/10 and even further at later stages.
Above are the maximum limits imposed on charging, which should never be exceeded.