Paul,
I am only guessing but I'd say 1-2 seconds for built-in caps, which is usually the minimum for add-on keep-alive modules. Unless your track is unsually bad or dirty, that should be adequate enough to cover any small interruptions in DCC packets.
Tom
https://tstage9.wixsite.com/nyc-modeling
Time...It marches on...without ever turning around to see if anyone is even keeping in step.
I note that some (all?) Wow Sound decoders have built in Keep Alive. I wonder how the related capacitance with the built-in KAs compares with the separate Keep Alives (KA1 - KA4). If significant capacitance that indeed is a great feature.
https://sites.google.com/site/markgurries/home/decoders/keep-alive-compatibility
In my case, I tried a Radio Shack 2200 uF capacitor in a Genesis GP9 that was a bit marginal on power pickup consistency. It was (barely) sufficent for the case of that loco and my track. So now I add a TCS KA or Soundtraxx Current Keeper to any marginal loco and that is usually a big help, since those units are many times the capacitance of my 2200 uF GP9 experiment.
I would guess that a moderate amount of uF would be a nice feature, trading off a bit with a slightly large space requirement for the decoder. Are others offering this built in and/or projecting that they will? My favorite decoder is the LokSound Select but I have seen no mention of including sufficient capacitance to help motor plus sound. LokSound does offer a separate device, like TCS and Soundtraxx.
Paul
Modeling HO with a transition era UP bent