I agree with thomasmalt that it is not wise but what you should do instead is have it so your users can set a cron time in a 'users' database table (like mysql) with columns: 'interval (INT)' and 'last_update (DATETIME)'. So every user has their own time set. Then you decide what is the lowest increment you will allow (such as 1 minute) and only let your users set times higher than that.
Then you run a cron every 1 minute which checks the 'users' table. Find any users where interval is less than or equal to the time since 'last_update'. Then run your actions for those users. This effectively does the same thing as setting crons for every user.