This is an activity I love to do at the end of the year. It's also a great tool for myself to reflect upon as I read the students' letters. I do this the last day of the year, and use the letters next year. I ALWAYS proofread the letters before even considering them for usage next year.
The last day of class I have students complete a survey and these letters where they write tips/advice to next year's classes. It shouldn't take more than 15 minutes. I also use these in the beginning of the year to complete my "Spanish Study Plan" that I have on my TPT page as well.