“I haven’t owned a watch for I don’t know how long”

“For I don’t know how long” is an expression for a long period of time, but there’s a joke:
 
“I haven’t owned a watch for I don’t know how long.”
 
The joke was cited on Sickipedia in January 2016.

 
Sickipedia
I haven’t owned a watch for I don’t know how long.
Joke added 3 days ago by hiraeth in Other > Wordplay
 
Twitter
no to the eu .
‏@brooking1980
I haven’t owned a watch for I don’t know how long.
4:52 PM - 17 Jan 2016
 
Twitter
Johan Hulaimi
‏@th1eves
I haven’t owned a watch for I don’t know how long.
5:44 PM - 17 Jan 2016
 
Twitter
Shit Jokes
‏@ShitJokes
I haven’t owned a watch for I don’t know how long.
3:37 PM - 18 Jan 2016
 
Twitter
LaffGaff
‏@LaffGaff
I haven’t owned a watch for I don’t know how long.
#funny #jokes #jokeoftheday
4:12 PM - 20 Jan 2016