When can we expect the first Platinum update?

It's already been issued...

We are currently awaiting the second update.
 
 

..which still happens to be the second update for what it's worth :) I can tell you this, Cakewalk will be releasing regular updates when they are ready and tested... I'm sure they will let you know when they are ready...

microapp
No, the 254 build was the official release.
254 may have been an update for the lottery winners who got the early release.
How about the next update ?

252  was the official release that I received on the 21st.

The 254 update came a day or 2 later...

microapp
If the update is going to be on a strict schedule, then one could maybe not schedule clients for a few hours that day, do the update and test thoroughly. If there were issues with the update, like breaking something unexpected, then you could roll-back.
This would become even more important if we actually were aware of what was going to be in the update.
Maybe the update would contain a fix for something of interest to a particular client which is not working.
You could push that client out until after the update.
A regular schedule would facilitate scheduling of downtime.

Unfortunately I doubt there will be a clockwork schedule (but I could be wrong), there are lots of variables. For example, there was a separate update recently of my amps to replace incorrect versions with correct versions. I suppose it could have waited, but as it was a mistake at Cakewalk's end, they wanted to solve it as soon as possible. I think "about monthly" will be the way it works. 
 
However, remember that you get to choose when to update. You can see what's in it, and decide to skip it, postpone, or wait until several updates have happened and update all at once. So the bottom line is you control the update calendar more than Cakewalk. 
 
As to what's coming up, I'm submitting a list of what I have planned so far in terms of content. This will be folded in with other features. Some features will not be announced too much in advance to avoid giving competitors a heads-up. Others will be announced well in advance.
 
Others will occur based on feedback. For example, the comments about the amp sims "get" that these are designed to be "plug-and-play" effects where you just drag them into place and start playing, and maybe make a few tweaks or put some other effects afterwards. The reaction could have been "This sucks, I want a rack of stuff so I can create my own amps, your presets suck, and you suck." In that case, there wouldn't have been any more amps.
 
But because the reaction has been favorable, today I proposed doing an "Amp of the Month," which I really hadn't considered before even though I have amps in various stages of development for my own use. If I finish them I'll have enough to last for at least a year. Probably the Acoustic Guitar amp, Hollow Body amp, Keyboard and EDaMp will be next...however even this brings up an issue of why scheduling is variable. The Hollow Body amp depends on when Gibson Memphis ships me several guitars for testing with it, so that might get delayed whereas the amp for EDM might get pushed ahead because I'm close to having it done anyway. 

21.0.254 was called Allston Update 1.

It was to fix a Melodyne use issue that cropped up during the lottery winners early use which was 21.0.252

Looking forward to the Demo version and hope it reflects whatever the latest update is when it's released.
 
If not, I hope it can be applied.