While there are too many variables to nail down specific conditions, proficiency comes with practice and familiarity. We keep our boat on a lift in an inlet where current is usually not an issue and our go/no go decision is based on bay conditions rather than wind conditions at our slip. Wind ahead or astern is no big deal and wind at 10kts or less isn't either. In my experience, winds over 10kts and more than 30 degrees off the bow or stern make backing in to the slip challenging. If you are going bow in, the approach is easier to control in higher winds with broader angles.
The bow and stern thrusters are great, but they don't have the force to overcome, or even compensate for, a significant cross wind (or current). Also, the pivot point of the R-29CB is right around the aft edge of the dinette table (aft of the pivot point shown in the factory line drawings due to loaded vs. unloaded considerations), so almost 2/3 of the boat's length is forward of the pivot point along with the majority of the boat's sail area. Along with the prop walk and prop wash dynamics when maneuvering at slow speeds, the boat's characteristics make backing in a cross wind very challenging, but the same wind in a head-in mooring approach is easier to compensate for. I have found getting out of the slip is easy, even in a significant cross wind, if you push yourself out with the boat hook rather than trying to motor out...way more controlled, especially with the ability to use the thruster remote when I'm in the cockpit.
So, like Brian said, no real answer, but I have found that experimenting in different conditions is best for developing proficiency, confidence and personal go/no go condition assessments. After a lot of practice with different slip/mooring conditions, the limiting factor for us is what the bay conditions are rather than our local inlet conditions. Our open water go/no go conditions depend on wind strength, direction and fetch. In general, on the Chesapeake Bay, we get 1 foot wind waves for every 5-7kts of wind when it has a reach of several miles. So 15kts and 2-foot wind waves doesn't sound bad, but add that to the normal swell conditions and take the short wave periods on the bay into account and it can result in suboptimal comfort conditions quickly. I'm sure others have different opinions, but here are my thumb rules:
1. No Small Craft Advisories (by plan...we have been out in "pop up" Small Craft Advisories which resulted in establishing this thumb rule...)
2. Good conditions if wave period divided by wave height is greater than 2
Good luck!