When to give notice

I’ve been on sick leave from my job since the beginning of the year. I’ve decided to quit once my paid leave runs out at the end of this month (March.) How much notice should I give the company? I don’t want to leave them in a lurch, but they have already been without me for quite a while, and I’m afraid that if I tell them too soon, they will just let me go right then and I’ll miss out on some of my paid leave.

Also, should I talk to the head of my department or Human Resources?

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