How quickly should a reliable Sri Lankan agency reply to emails?

A pro-level Sri Lankan travel agency will usually acknowledge and answer your first email within 12-24 hours on regular business days, not after.

How fast they communicate is a solid signal of organisational competence. With the time difference from Western markets in the mix, a reply on the following day is a normal expectation. If the agency goes past 48 hours to respond, and they did not include an out-of-office auto-reply, then it really points to them not having the back office setup needed for tricky logistics or to respond to on-tour crises.