Depends. The cpu attempts to hold the maximum possible speed on any cores that are in use.

On my water cooled and specifically tweaked desktop- yes. It’ll hold max boost indefinitely, even with all threads. (getting to about 80c after 10 mins). Single-thread max is faster, and it’ll hold that as well.

My laptop will pull power within 15 seconds and be down to base clocks in a couple mins. Unless I set it down outside and it’s very cold.

It depends on motherboard and cooling. 6700K, for example, is constantly running at 4.2Ghz or 4.5Ghz (winter clocks). Constantly while thermals allow it... Non-overclocking motherboards allow it to boost for 2 minutes, after that, it's iffy.