Employee stock options

It should meet the exercise/ vesting period. Then they can dispose them.

How long before an employee should have worked in an organization before being eligible for the employee stock plans?

I think it s usually one or two years 

Is there a fixed period one is suppose to complete?

It’s actually the comany wish. Sometimes years and sometimes they give to managers only.

okay? is there any amount fixed for the employee stock options which are given every year? Or this is also just on the company's wish?

The employee can subscribe at a fixed price or at the fair value of share.how many he can get depends upon the company policy 

Okay Thanks for the information

No problem, unlike equity forfeited re issue or allotment which is done on pro rata basis, eso standard doesn’t have any policy while issuing shares. 

What is it like on a pro-rata basis?

That is one way of allocating shares between all applicants. Go through this link- https://www.toppr.com/guides/principles-and-practices-of-accounting/issue-forfeiture-reissue-of-shares/pro-rata-allotment/

I also have read that applications are accepted on FIFO basis as well.