Question: Are both *Als* and *Während* possible terms in this sentence? Do they both express that the actions happen at the same time? Although the waiting is longer..

> 
> Als/Während er auf den Bus wartete, bemerkte er, dass er kein Geld dabei
>  hatte
> 
> 
>

Answer: 
In your sentence both are fine. However, in your very example I would tend to use *als*.

You normally use *während* to indicate that two events or activities are happening at the same time (and for a while).

> 
> Während er auf den Bus wartete, hörte er Musik.
> 
> 
> 

An example that is in stark contrast to the one above is when an event interrupts another. In that case you go with *als*.

> 
> Er wartete gerade auf den Bus, als er die Explosion hörte.
> 
> 
> 

(Note that the conjunction is part of the "short-term event"-clause.

So, my point so far is that you use *während* for parallel long term events or activities and *als* for a short term activity that interrupts another. Coming from this perspective, your sentence must be:

> 
> Er wartete auf den Bus, als er bemerkte, dass er kein Geld bei sich hatte.
> 
> 
> 

However, *als* and *während* are a bit more flexible and are not restricted to these use cases. You can also replace *während* with *als* for two events happening at the same time.

> 
> Als er auf den Bus wartete, hörte er Musik.
> 
> 
> 

And when combining a long-term event and a short-term event, you can use both conjunctions as part of the "long-term event"-clause.

> 
> Als er auf den Bus wartete, hörte er die Explosion.  
> 
>  Während er auf den Bus wartete, hörte er die Explosion.
> 
> 
> 

The latter one (with *während*) is technically a bit incorrect but you might hear that, though. The former one is definitely correct.  

When using *als* think of the event being kinda shrinked to a single action without any relevance of duration. So you end up with having to parallel events with undetermined duration that just happen to be at the same time.

(Note that the last paragraph is not a scientific explanation but rather for the sake of better understanding.)
"Als" can be translated as "when." Während can be translated as "for" or "during."

"Als" refers to a "point in time." Während refers to a "span" of time, or time **period.**

It is when referring to "short" periods of time that the two overlap.