It is down to the employer to set out the rules for how holidays should be requested.
Setting out the rules in writing, in a Staff Handbook or holiday policy for example is good practice.
If you have not set out the rules in writing, then the minimum amount of notice you can expect is at least twice the amount of time that is being requested.
For example, a 2 week holiday request should be submitted at least 4 weeks prior to the dates of the holiday.
Of course, this may not be practical for your business, as you may require more time to plan, therefore setting out the rules in writing will benefit you.
Think about how you want your staff to request their holidays; should they fill in a form, send you an email or use your online HR system if you have one in use.
Above all, be consistent in how you expect your staff to request holidays and give yourself enough time so you can plan around staff holidays and ensure you have enough cover.