I will be granted an H1 visa to work in the US as a part of my onsite assignment from the employer. What all are the eligibility documents required to file an H4 dependant/spouse visa? What is the procedure for this application? What is the average processing time?

Thanks in advance :)

It depends if your spouse is in the US or outside the US. You must be

If inside the US, then form I-539 to change status, with a copy of your I-797 approval notice and your marriage certificate.

If outside, she applies for the H-4 visa with form DS-160, your I-797 approval notice and your marriage certificate.