If performing training independently it is recommended to use the criteria listed in the following tables as guidance.

Tab. 1: Criteria to evaluate single sets/intervals
The physical load is appropriate if:

self-perception according to Borg (respiration and exertion) is less than 7 at the end of a set
no dizziness, discomfort or the like occur
no feeling of pressure in or at the rib cage or head, no pinching pain or feeling of pressure in the heart area occurs
no muscle or joint pain or the like
saturation more than 90 (terminate exercise if saturation less than 88)
Reduce or terminate exercise when:
One or several of the aspects listed above occur: termination of exercise or building in breaks which are long enough to ensure safe performance. In general, it is possible to perform the single sets distributed throughout the day instead of performing them all at once, especially when you have just taken up training again.
Start the next set, if:

Sättigung mind. größer oder gleich 90 (ideal wäre größer oder gleich 95)
saturation is at least 90 or higher (95 or higher would be ideal)
self-evaluation according to Borg (respiration and exertion) is 3 or less
respiratory rate(number of breaths taken in 10 seconds) is less than 5
breaks last 2 minutes minimum
generally in a good state (relaxed muscles, breathing goes "all the way down" and easy)
Tab. 2: Criteria to evaluate the entire training unit ¹
Criterium Evaluation Consequence

You hardly feel any physical exertion or fatigue.
You could perform the same all over again without a break.
Physical underload

hardly any training effect

one level up

You feel older than usual, tired and you have heavy legs.
You feel the need to rest.
You feel that you have 'really' worked.
Recovery period is relatively short and you quickly regain strength.

one level up

Pressure on your joints
You have aching muscles, other pain
Increasing COPD symptoms (cough, sputum, breathlessness)
Recovery period is long and you need a long time until you regain strength.
Physical overload

possibly dangerous
one level down

or skip a training unit

¹ Having an infection / an exacerbation: no training. The same applies to a period of time in which you are undergoing cortisone shot or if taking antibiotics.