TL;DR: it's complicated.
Memory Alpha has a long section on multiwarp speeds. In TOS and TAS, going beyond warp 10 seems to be perfectly acceptable: the Enterprise reaches warp 14.1 in the TOS episode That Which Survives and warp 22 in the TAS episode The Counter-Clock Incident, while warp 15 is mentioned in the TOS episode The Changeling and warp 36 in The Counter-Clock Incident. According to the book Star Trek: Starship Spotter, the redesignation of warp 10 as infinite speed occurred in the year 2312. The warp factor specifications before 2312 were rated by Starfleet using the Original Cochrane Unit (OCU) warp scale, while warp factors after 2312 use the Modified Cochrane Unit (MCU - not Marvel Cinematic Universe!) warp scale.
But to answer your question about transwarp speed specifically, the best I've found is the following:
In the October 1995 issue of OMNI, science advisor Andre Bormanis stated the idea of warp factors beyond 10 in the alternative future was in a recalibration of the warp scale, as ships had gotten faster. Possibly warp 15 was set to be the transwarp threshold instead, according to Bormanis, and warp 13 in that scale would have been the equivalent of warp 9.95 of the previous scale.
According to Star Trek Encyclopedia (3rd ed., p.555), warp 13 from All Good Things... may also allude to some type of implementation of the Federation transwarp drive technology from VOY: Threshold.