How soon can we apply for H4 if F1 is rejected

[shri2400]

Hello,

My fiance will be applying for a F1 visa to travel as a student. If for some reasons, the visa is rejected how soon can I apply for her H4 so that she can travel with me to the US [Is there any timeperiod you have to wait before you can apply for a different visa again]? I am on a H1B visa working in the US

Thanks

[immihelp]

Can apply at any time, as soon as the appointment is available.