Questions that relate to the academic rank of assistant professor.
In relation to US academia the following is stated from wikipedia
The rank of assistant professor generally is held for a probationary period of three to seven years, after which the individual will either be promoted to associate professor and granted tenure (i.e., can not be fired without cause and a formal hearing process) or will be terminated from employment. As of 2007, 23.1% of academics held the rank of assistant professor.
Other countries that use the rank can be found here