Hot SLK

How long should an SLK engine stay hot? Specifically, a '98 SLK 230 (which I bought two days ago).
I spent 4-5 hours driving around in local traffic, normal stop and go.
I used the ac a little bit, but mostly not. The outside temperature was probably about 80 F when I started, and 67 F when I got home at 10:00 p.m. and parked the car in the garage.
Checked the car just now. The engine and radiator cap are still hot to the touch - FIVE hours later! Is this normal??
BTW - if it matters - the oil and filter were changed when I bought the car, i.e., two days ago, using synthetic oil.
Thanks as always.
--Jonathan
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