How much incline is too much?

[skidoo]

"OOPS!" is too much.

Yes, I woudn't take my eye off the terrain for a second to look at an inclinometer. But, I may ask my passenger to read one to me...

 

[TripleR]

When running near your max comfort angle you also need to consider if the upper wheel goes over a bump or the lower wheel drops into a hole or soft ground. Either one increases the tilt angle and the faster you are going the greater that increase.

Agreed and another thing to consider is air pressure and condition of tires. I have seen tires come off the rim under load and this would be disastrous.

The incline on which you can mow "safely" is going to vary from one model to the next. My tractors can handle more of a slope than my nerves can.