Malla:
Its always at the instructors descretion working of coarse within the boundries (if any) set by CFI or the company, the main governing rules are those basic VFR rules set by the Authority in your country but as we know they are far more generous than that you would be happy sending your student into.
Our limits are the Basic VFR rules and then of coarse its down to the instructor to decide if appropriate for even that particular student.
Setting limits to take away decision making cost money ie: min viz 8kms and so its 7kms and hes only flying local but hes grounded. Even if you follow company limts and send him solo its still you who is responsible.
cheers: