Do you mean you want to know (A) how many seconds it was since the previous login, or (B) how many seconds each session is?
Date-time management is an area where many DBMS products are far from standard compliant! The SQL statements below are both ANSI/ISO standard compliant:
(A)[tt]
SELECT (login - (SELECT MAX(login) FROM loggtable AS l_sq
WHERE l_sq.user = l_main.user
AND l_sq.login < l_main.login)) SECOND(10)
FROM logtable AS l_main
[/tt]
(B)[tt]
SELECT (logout - login) SECOND(10)
FROM logtable
[/tt]
Note that USER is a reserved word in ANSI/ISO SQL. To avoid future problems when upgrading your DBMS, you'd better replace that column name, or double quote it (i.e. "user").