I disagree with the answer given. Both for and during translate to durante regardless of the number of the following noun. The main difference between both terms in English is that for is generally used with random periods of time, while during is generally used with established or "official" periods of time. That's why in your examples during is used with February, and for is used with hour/years. Even though hour and year are both official and well established units of time, the hour or the years described aren't different from any other hour or set of years, while February is a specific month. Now, Spanish durante doesn't have that limitation: it can be used with any kind of period of time, be it established or not. Thus, you can say:
Durante el verano suelo ir a pescar. (During summer I usually go fishing)
Estuve hablando durante una hora. (I was speaking for an hour)
Durante los fines de semana, el tráfico es menos denso. (During [or on] weekends, traffic is lighter)
Durante milenios no se supo qué causaba la lluvia. (For millenia people didn't know what caused it to rain)
As you can see, the number of the object doesn't have anything to do with using durante or not. Being speculative, the reason why your article says so may be because there are many examples with established periods of time that are used in singular, and not so many that are used in plural. Maybe, the author tried to derive a rule from the examples he or she could think of, and reached the conclusion that number played an important role. In any case, that is not true.
The most common case where you use for in English is with present perfect. The use of present perfect is quite clear-cut in English (at least compared to how it's used in Spanish). In Spanish, however, present perfect is an ill-defined tense, in that its usage varies a great deal from a region to another. In some Latin American countries (I can't put my finger on specific ones), the present perfect is hardly ever used. For example, consider things that just happened:
A (on the phone with B): Where are you?
B: In the park.
A: I've just been there, but I didn't see you.
A (al teléfono con B): ¿Dónde estás?
B: En el parque.
A: Recién estuve ahí, pero no te vi.
As you can see, it's isn't necessary to use a perfect form (recién he estado ahí), even though you're describing something that happened immediately before. The past simple (estuve) or some periphrasis (acabo de venir de ahí) would be more natural-sounding options.
Moving on to Spain, you can see the opposite phenomenon: the present perfect tense is used even when you specify a time reference that clearly belongs to the past:
I saw the most boring movie yesterday.
Ayer he visto un tostón de película.
This is a perfectly natural thing to say in Spain, although most Latin Americans would probably cringe if they heard such a wording.
Where am I getting at with all this? My point is, in English using the present perfect tense immediately tells you the action described is intimately connected to the present. Paired with for, the only logical implication is that the time specified extends to the present. In Spanish, however, the present perfect is much more vague and doesn't necessarily carry this connotation, so saying things like He estado trabajando durante dos horas doesn't necessarily mean you're still working, while I've been working for two hours always does. In these cases, much better alternatives are llevar + gerund or present (simple or continuous) + desde hace, that is:
Llevo trabajando dos horas.
Estoy trabajando desde hace dos horas.
In short, Spanish durante is actually a quite simple term to grasp: just use it whenever you would use either for or during in English, keeping in mind "present perfect + for" structures would be better translated by paraphrasing the original sentence rather than translating it word by word.