Trust me, you will know. If you start to feel pangs of tendinitis then you know youre starting to push too hard. Heed this warning or you could more seriously injure yourself and set your training back by weeks or longer.
But I don’t think its as cut and dry as “tendons need 2x as long as muscles”. If you’re not sure, the safest thing to do is not do anything too extreme for you. If you’re worried, I’d say if you can’t do at least 5 or 6 reps in a single set (fully rested), then don’t do it in a workout. If you push it hard you have to be very comfortable and confident with your body’s abilities and movements so you can recognize the line between pushing hard and injuring yourself.