#1
I have been playing with a crooked wrist with my thumb pressing hard into the back of neck pointing towards the head for around 10 years and it has caused me to develop tennis elbow (or so I think).

A guitar teacher has told me my wrist has to be straight with my thumb behind the index/middle finger and has told me to do a chromatic exercise to help get used to it.

I am finding this very difficult to play and I understand that it is going to take some time to adjust. I am interested if anyone else has had to make this change and how long it took them, any exercises they thought were more helpful than others etc.

I play rock/lead if your interest.

Cheers
#2
Make sure not to make the same mistake with your thumb pressing into the neck again, you should keep your hands as relaxed as possible.

It won't take too long to get used to the change if you keep practicing. The exact amount of time it takes depends on a lot of factors and isn't really worth trying to work out when you could just practice knowing that it will get better.