It will probably have stopped receiving them if you've gone many months without getting any. I doubt that Samsung will formally announce that there won't be any more, but as the S5 must be 3.5 years old now I'll be a little surprised if they are still issuing them for that phone.
When it needs to be replaced is a separate question. You don't have to replace a phone the moment those stop. If you are sensible with how you use it (mainly what you install) it's unlikely you'll run into a major problem. I used my last phone until it was 4.5 years old, and it wouldn't have had a security update for the last couple of years. But it's your choice.