Hi, It is very common for employees to follow up for an increment with HR with some hope. Have you communicated to the employee at the time of joining that he/she is eligible for an increment only after completion of one year? Not sure.
In case of such a request from employees, you should educate them that as per the Company's process, the increment will be processed after one year of service, even after the completion of the performance appraisal. Give them hope and also the tentative date/month from which the increment will be awarded.
The amount of hike that can be given is at your Company's discretion based on the performance of the employee and the Company as a whole. The normal industry norm is 10-15%, but it can be less. It is company-specific.