Not sure you can even do that easily. There is no relative date feature. You'll probably need to calculate the date with a CB Query Field then use that in your conditional. Example as follows.
Code:
SELECT COUNT(*) FROM `#__users` WHERE `lastvisitDate` <= ( NOW() - INTERVAL 1 MONTH ) WHERE `id` = '[user_id]'
The above example will return 1 if the user last logged in less than or equal to 1 month from the current datetime. It'll return 0 if the user logged in within the last month from current datetime. So for example you'd use the below conditional.
[cb:userfield field="cb_queryfield" /] Equal To 1