I hope this message finds you well. I am a licensed attorney with over a decade of employment law experience. It is a pleasure to assist you today.
I typically advise that an employee can accumulate at either 1.0 or 1.5 days of vacation per month (you must work at least 40 hours per week and an earned vacation day can count as a day of work towards accumulation), when they have worked a full month. I think it is good to not allow accrual until they have been there for 90 days (a probationary period) and I would advise that the start date for accrual and reference relative to vacation will be that 90th day. It makes things a little more complicated, but you don't have to worry about giving back accrual of time though. In short, it evens out.
I also recommend that you only allow the use of vacation time no more than 5 consecutive days (one business week) and that it must be used within the calendar year...i.e., it does not roll over to the next year. Because people will attempt to abuse this at the end of the year, it is a good idea to allow the use of no more than 5 vacation days in total during the month of December.
Most reasonable employees will space things out once those policies are in place, in total.
There is no magic formula here, unfortunately. Some of this depends on the fiscal strength of the company, etc. One thing to contemplate also is that you must give a certain number days notice before using a vacation day and you may want to limit the number of people that can be off at one time in a particular division of your company such that you are not crippled by absence. In that instance, it is first come, first serve. If you use that approach, make sure that requests are submitted to the supervisor in writing...preferably email. That way you can confirm who asked first.
Please rate my answer positively and let me know if you have any other questions or comments.
Best wishes going forward!