From here on Theoryland:
Is there a complete language of the Old Tongue, and if so how long did it take you to develop it?
There are basic 880 some words—maybe 900. I got a list of what is considered basic English—which are the 800 odd words of a basic English vocabulary. Removed the words that were of no use in the context of my world. Came up with words in the Old Tongue in each of those English words and then added those words that did have a specific context in my world.
Elsewhere on that page is an interview where he said this:
What language is the Old Tongue based on? Gaelic, Russian, Spanish, Chinese, Japanese, Arabic and some additions of my own—bridging material, if you will. Grammar and syntax are a blending of English, German and Chinese, with some influences from a set of African languages, read about long ago and all but the oddities of structure long since forgotten. There are inverted constructions, for example (as in Mordero dagain pas duente cuebiyar!—literally, "Death fear none holds my heart!") and places where the article is omitted, especially where the word is a title or has gained enough importance to now incorporate the article; the absence of article indicates that it is the important or special meaning of the word that is intended. Though even then, it is not a hard and fast rule; the same inconsistencies of English are incorporated here. I am attempting to create a language which has grown, not one which was made.
It seems to be a complete enough language in terms of grammar and authentic enough for Jordan to use, though lacking in vocabulary. From these quotes I get the feeling that he created the language well enough at one point that he didn't have to spend hours conlanging to be able to put a few sentences into his novels.
In the Wheel of Time Companion there was an English-Old Tongue dictionary, which is uploaded on Tor.com here. At the beginning on the website (not sure if this is also in the book as I don't have it on me) this is written:
While it is reminiscent of Tolkien’s ground-up creation of Elvish languages for The Lord of the Rings, The Old Tongue differs in that Jordan took a top-down approach, creating a dictionary of terms from which some basic suffixes, roots, and usage rules emerged.
I didn't bother scrolling all the way down or counting the words, but it doesn't seem to have much grammar aside from the suffixes and stuff. Although we don't have access to this, Jordan seems to be implying that the grammar does exist. All I could find was this on the Tar Valon library.