I was able to get it to work with this query:

SELECT
round(
(
(SELECT unix_timestamp(signupdate) FROM users WHERE ipaddress = "10.10.1.100" ORDER BY signupdate DESC LIMIT 1)
-
(SELECT unix_timestamp(signupdate) FROM users WHERE ipaddress = "10.10.1.100" ORDER BY signupdate ASC LIMIT 1)
)/60) as elapsed
FROM users
WHERE ipaddress = "10.10.1.100" LIMIT 1;

i doubt that it's the most optimized way of doing it but it works..

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