In my day-to-day experience, I would suggest an average turn around for increased tie-pairs is approx. 3 months. My very own Cab has recently undergone the same process.
The comments are based on what I would call 'High-demand' Cabs, ie: those that are a great distance from the Exchange, thus having historically slow ADSL speeds.
As an engineer, with these Cabs you can see the capacity draining quite quickly, knowing full well that expansion will be required imminently. As I say, it would appear the average time from full to available would be around 3 months, but like your good-self being unable to quantify the wording, Walter, I only have my own personal findings to go off.
Before the debate begins around, 'Why don't they provide 2 x 96 ports on these Cabs, instead of just the one', I kinda understand that train of thought as a local engineer listening to the EU's demands as the build up to their Cab going 'Live' approaches.
However, these Cabs are planned from all over the country. The one I reference to above (in Lancashire) may have been planned by someone down in Cornwall. He/She will have no idea of local demand. That said, OR can NOT work to if's, but's and maybe's ........ there's plenty of folk who don't put their money where their mouth is when it comes down to it. So, it will usually be found to be a 1 x 96 ports feed* at the initial stage.
*This is based on my on findings, and there have been a few instances when a brand new Cab has had more capacity supplied at the initial stage.