Add this template to your user page, with the year, month, and day your account was created. If you don't know the day your account was created, see your preferences here.

Sample code Resulting infobox
{{User Wikipedian for|year=2001|month=01|day=15}}
This user has been on Wikipedia for 23 years, 10 months and 22 days.
{{User Wikipedian for|year=2001|month=01|day=15|link=true}}
This user has been on Wikipedia for 23 years, 10 months and 22 days.
{{User Wikipedian for|year=2001|month=01|day=15|diff=http://en.wikipedia.org/w/index.php?title=Special%3ALog&type=newusers&user=&page=Example}}
This user has been on Wikipedia for 23 years, 10 months and 22 days.
Adding a serial comma:
{{User Wikipedian for|year=2001|month=01|day=15|sc=y}}
This user has been on Wikipedia for 23 years, 10 months, and 22 days.