what do you mean "digits"? numbers?
you need to add Change event to your textbox and
count the digit there.
see this demo, open Form1 and type anything
in the first textbox.
If you mean that the field in question will ALWAYS be numeric and you are looking at its length, there are other questions to ask. Such as... Will it involve decimal points? Are you using the "thousands" separators (either comma or dot depending on regional settings)? Do you have mixed digit and alphabetic characters and you are trying to determine the count of digits in a mixed field? Please specify what you seek.
Digits = Numbers, Characters = anything. That seems to be what is causing the confusion. Regardless, You would use the Change event of the textbox. This fires once for each each character typed as arnel suggested.