I see that inshaala asked,

...

Quite a few questions ...

All of his questions could come down to a single answer ...

What's reasonable? ... **Whatever both parties agree to do. **

But, as a starting point ...

1. when they 'want to take a break' ... you should establish a specific
   schedule -- you can't run over there on 5 minutes notice, can you? 
   You can't sit home waiting at the telephone can you?  OTOH, if you're
   not there when they want/need you, the relationship will get old/tired
   very fast. 

2. Copyrights ... AFAIK, if you do the work under their name, they would
   own the copyright.  But they might give credit on/with the image to the
   photographer ... do they already do that now with their own work? 

3. Payments ... if they are small and just starting out, they might not be
   able to pay a regular salary ... OTOH, a barter system might be perfect
   for both parties ... e.g. 2 hours of your time minding the front desk might
   be 'repaid' as 1 hour of use of the studio for your own work.  Keep in
   mind that if you use the studio to bring in portrait work, they might prefer
   that 'they' do the work and pay you a portion of the total fees as a finder's
   fee or commision.  Don't forget to keep good records. 

4. An additional idea ... Although I don't know the ofice layout, I suspect
   they have a front desk or entry area or waiting room.  It might be a good
   idea to set that area up as a mini gallery ... with the selection of images
   changing every month; credit would be given to the photographer who
   is showing; you would have a notice of the next scheduled 'show'.  They
   could even allow other artists to show there. 

I know that this works.  In the early 1980s when I first worked for Deloitte,
Haskins and Sells (now Deloitte & Touche [USA], or Deloitte Touche Tohmatsu
[Global]), I used Modern Age on Vesy Street to do my film and photofinishing. 
They always had images on display in the front office/sales desk area.  One
day I asked how they decided who would be on display ... the answer was,
"I decide if they are good enough." (I had addressed my question to the
manager) He then asked to see my portfolio.  As I didn't have one, he told
me to bring in a selection of what I considered to be good -- 2 days later
I asked what he thought and he replied, "You're next month's show."  It was
a bit of a scramble to select just 30 images out of the landscape/nature
photographs I had shot over the previous 15 years ... and now I do have
a portfolio. 

Best of luck ...

HTH

--
Martin