How do I get the textbox to enable after a certain amount of text is inputted? - vb.net

Question

So my next question(i know i know ive had a lot of questions already but im learning and my teachers suck)
but I am trying to get the textbox to go to readonly after a certain amount of text has been entered. I know how to make it a read only textbox but only after Ive had one set of data entered. i need it to be readonly after 7 days of data has been entered
I've tried inputtextbox.enabled = false
'Validating if user input is a number or not
Dim output As Integer
If Not Integer.TryParse(InputTextbox.Text, output) Then
MessageBox.Show("ERROR! Data must be a number")
InputTextbox.Text = String.Empty
Else
UnitsTextbox.AppendText(Environment.NewLine & InputTextbox.Text)
InputTextbox.Text = String.Empty
End If
InputTextbox.Enabled = False
I'm expecting it to disable after the user has entered 7 days worth of data but it only disables after one day of data is entered

Answer 1

Since the entries to UnitsTextbox are all done in code, this TextBox can be set to read only at design time.
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
Dim output As Integer
If Not Integer.TryParse(InputTextbox.Text, output) Then
MessageBox.Show("ERROR! Data must be a number")
Else
UnitsTextbox.AppendText(Environment.NewLine & InputTextbox.Text)
End If
'Moved this line outside of the If because it happens either way
InputTextbox.Text = String.Empty
If UnitsTextbox.Lines.Length >= 7 Then
Button2.Enabled = False
End If
End Sub

Answer 2

Here's some simple psuedocode
Private Sub InvalidateTextbox(sender As TextBox, e As KeyEventArgs) Handles TextBox1.KeyUp, TextBox2.KeyUp
'FOR ANY TEXTBOX YOU WANT TO CONTROL WITH THIS SUB, ADD AN ADDITIONAL HANDLE.
If Strings.Len(sender.Text) > 7 Then
'^SIMPLE CONDITIONAL, CHECKING IF THE LENGTH IS MORE THAN SEVEN CHARACTERS, MODIFY THIS TO SUIT YOUR NEEDS.
sender.Enabled = False
'^IF THE CONDITIONAL IS TRUE, DEACTIVATE THE CONTROL, IF THAT IS WHAT YOU ARE LOOKING FOR.
sender.ReadOnly = true
'^IF YOU WANT READONLY,NOT ENABLED/DISABLED.
End If
End Sub
This code will execute every time a key is pressed while the text boxes are active. What is after "Handles" defines what events will trigger the sub.
sender becomes the textbox object that triggered the sub. e holds all the event arguments for the keyboard, so you can evaluate things like which key was pressed and other neat things.
There was some confusion on if you wanted enabled/disabled or readonly, both options included.