I have been off the pill for 13 months and still have no regularity to my cycle. It has ranged from 45 days to as little as 31 days.

I am now up to 51 days since the first day of my last period. I have tried HPT - which came up negative.

How long should I wait until I test again or go to doctor?

I think you could possibly be experiencing anovulation,
Here is a link to give you some further info.
I think your doctor needs to be consulted as this medium is not conducive to make a true diagnosis.
Anovulation - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Anovulation)

Definitely go to an ob-gyn.

However, it's possible that your longer cycles were failed pregnancies. This is not unusual, and IF that's what is happening, it is a good sign. Most people don't know this but a large proportion of pregnancies end very early. It's a natural process. When I tried to get pregnant, at first I made no attempt to time intercourse. Later, I did and that helped a lot. But I had about 8 "long" periods that were probably pregnancies that didn't work out and ended within 3-4 weeks of ovulation. (When I was not trying to get pregnant I was VERY regular.)

On the other hand, if your cycles are really this irregular, then you should consult a doctor. Actually, you should consult one anyway.

What is your age?
And did you have regular cycles before you went on the birth control pill?
Also, have you been timing intercourse to come one to two days before ovulation?
Have you taken your temperature or otherwise tried to figure out when you ovulate?

Answers to all these questions would be helpful!