We have a listed barn that we are converting and have had to put lime render on the inside of the external walls to allow the structure to continue to breath.
The guy doing it has just told us we must leave 6 weeks between applying the lime putty final coat and decorating the wall (obviously with breathable paint) even though there has been at least 5 days between each of the first 2-3 rough coats.
Being a novice at lime, can anyone advise me if this is correct and if there is a way of testing the wall just in case it dries quicker in this nice weather as we are supposed to move in 7 weeks and the lime work is not all finished yet.
Thank you.

The best time to put on the next coat is when the earlier coat is what's called 'green hard ' - which is hard enough so that you can't dent it with your fingertips, but you can score the surface with your nail. That usually takes two or three days with an 8mm coat, maybe a bit longer with thicker coats on irregular stone walls and in rainy, humid conditions. Thus I tend to wait a week between coats, but six weeks is taking the wotsit! Unless, as Biff says, your plasterer has an interim job lasting, say, six weeks!

Here in the Czech Republic, they leave things to dry out for a year. It's quite common to see new houses without their final layer of plaster & families moved in. Maybe their builders have a lot of other work to go to...

Oops, I also misread the post! If you are going to use limewash you can apply it three days or so. Otherwise, wait until the plaster turns pure white (which may only be about a week anyway, if the putty/silver sand coat is just a 2mm skim).