What is a safe operating temperature for a CPU under heavy workload?

I would like to know if modern CPU's automatically adjust their workload to current temperature

The CPU can't adjust its workload - it would be up to the operating system scheduler to do so.

What CPU's can (and will) do is begin to throttle their operating frequencies down when the temperature of the cores gets too high.

From this forum post:

If temperatures increase beyond Hot Scale, then ~ 5c below Tjunction Max, Throttling is activated. The Digital Thermal Sensors (DTS) are used to trigger Intel`s TM1 and TM2 technologies for frequency, multiplier and Vcore Throttling within individual Cores. If Core temperatures increase further to Tjunction Max, then Shutdown occurs.

From this Intel Datasheet for the Core 2 Due (June 2009):

When Thermal Monitor 2 is enabled, and a high temperature situation is detected, the Thermal Control Circuit (TCC) will be activated. The TCC causes the processor to adjust its operating frequency (using the bus multiplier) and input voltage (using the VID signals). This combination of reduced frequency and VID results in a reduction to the processor power consumption.

The real safe operating temperature is documented in the data sheet of you CPU model - there are some variants o i7 CPUs

But this kind of CPU is pretty good in managing it's own temperature.
It can even run parts of the CPU faster than nominal if heat output allows, and changes the speed then other parts get active, also producing heat.

So, as long as the cooling system (like a fan) works normaly, there is no need to worry about that.

Temperature measurements of a cpu can be taken in different locations, resulting in different values.
Two common measurements are at the CPU case, and at the actual CPU silicon die inside.

For example, a i7 could handle a die temperature of around 100°C or so, and start to throtle down at something like 105°C to 110°C.

For real numbers, see the technical data summary table or datasheet of the CPU you are using:
5th Generation Intel® Core™ i7 Processors - Product Specifications

Assuming it is the 5550U i7

Max temp is 105c

Source of information