I realize each horse and each trainer is different. With that stated and accepted, how long do you expect it to take to add enough speed to be in a D? Any D? Seriously. I'd like to hear personal stories and experiences. 

that all depends on the starting point  

Pretty darn broke with 30 days of prior barrel work. I'm happy with trainer, just wanting a ballpark figure in months for when to expect to be competitive on a weekend warrior senior citizen basis.

Are you talking about with the trainer riding the horse or owner?  I sent one to the trainer that was super broke the way she likes them and the first barrel race he was in he placed in the 2D. That has been a long time ago but I think about 6 weeks. He went on to be a reserve world champion in poles, top 10 in barrels. I got him home and he never had to actually run again!  His job is to take care of me. He is the paint in my avatar

I've patterned a few that I never entered, just helping friends out. And patterned a couple of my own. It's going to depend on the horse and its confidence. Some fall apart when you add speed and some it's like their born knowing. Some in between. I've had one of mine take a year to start getting 4D money and I've had a couple that were able to pull a 4D check while adding speed and falling apart after 30 days. One was mine and one was passed back to the owner by this point, I just started doing lessons to teach her how to fix him when he did fall apart. The faster you ram jam them to get into the 1D I think the faster and more prone they are to falling apart. But if they hold their confidence and you just keep building on it I've seen some really turn on quickly, like 2-3 months. The second I have one seem a little mentally shaken because it got its feet bound up or whatever, to where you can see that confidence crack just a little, I back off. Some will push through and keep going and let the horse figure it out which either works or ends badly.

When people get in a bind and want to rush I tell them to go watch the 3 year olds run the juvenile at BFA. A lot of them have been on barrels for a year or more and still can run like its their first time ever and not clock at all. And then you have some freaks of nature that are just born to do it in their sleep. I don't think there is really a good average time, everything changes with each bloodline and trainer style. But I would say if you have a good trainer, that can read them well and know when to back off or push them, they could clock in 3-6 months after the first time on a pattern. (When I say clock I mean at least in the 4D money zone or better.) Obviously you'll have some outside of that time frame either way, but I'd say thats a decent overall blanket timeframe. 

Editing to add after seeing maybe senior rider, make sure they're confident enough for you in their ability. If you're more of a passive rider or aggressive, make sure they fit into your level of confidence as well. They may clock in the 4D after 2 monts of training, but also might still need you to help them a lot. That factors in to training time too, seasoning time, etc. Its kind of how automatic do you want it to be. That is a point for all ages and levels, do you want a horse that a monkey could pull a check on or do you like the ones that make you work for it. Or in between. 

I'm kind of in the "monkey" range.