Do you mean how long do they take to fully "open" up or do they sound similar as they do? If the first then you reckoned on 20 years plus for mahogany and I'm not sure there are many Khaya topped guitars that old out there. Todd made a Khaya Weiss so he may have some views - the only one I've done is 30 miles away with Martha but she says it's sounding sweeter every day. As usual it's going to depend on the piece of wood - mahogany is a little better know but I believe there are around six different sub-specie/variants that are sold within the generic "Khaya" name and within each of these there will be different woods depending on the tree/cut etc.
I'm in the process of making a little all Khaya tenor guitar so I'll let you know how that develops.
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Dave White
De Faoite Stringed Instruments". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010