I would suggest its at least the distance of the height of the building, if If you premises and environment allows you to do so of course . 

A large area / space that allows good levels of control to accommodate everyone safely is critical 

Hi Steve

Nothing set out in stone and I don't think the height of the building would be much of a parameter UNLESS a fire was likely to result in the building collapsing - and if that was the case then the building should probably be condemned as being structurally unsafe.

My last workplace was in an 8 storey building. To the front one way street West-East giving access from a motorway to a city centre. To the back one way street East-West giving access to said motorway, so both very highly trafficked.

Our various meeting points were on the other side of each of the streets to the sides of the building. Typically took about 5 minutes for everyone to egress the building and get to their muster points, partly as crossing the side streets presented its own traffic risks. Traffic lights at two corners, but in practice many people needed to cross one of the side streets away from the traffic lights due to the volume of people to be evacuated.

If you took this to the extreme and considered some high rise in London, the muster points would be a long way from the building if greater distance than the height of the building and there would be chaos.

As has been pointed out, there MIGHT be occasions where the emergency services would dictate that people move further away from a building, so beyond the initial muster points.

....and it might be appropriate to have different muster points to deal with a bomb threat than fire.

I am not sure distance from the premises is the main consideration 

The biggest issues I have found is insufficient space at the assembly point or the assembly point in a dead end, making it necessary for people gathered there to walk back past the building theyve left to get further away!!

But why even gather at an assembly point??? If you are operating a sweep method to account for staff, do you really need to gather all staff togther..... outside....in the rain????? I worked on an evacuation plan for a large city centre office with several thosand occupants. With no suitable open space nearby we adopted a system where only key staff would gather. These would be those who may be required during an emergency such as Estates, Security, IT and some Managers. Everyone else dispersed and did their own thing We established an emergency information area on the company website (hidden from customers). Lastly we used a answerphone type message system for those without internet access or if the web fell over. That information and text system could be operated from another site if necessary. The information displayed would say the building was closed and give an estimated time how long it would be unavailable for. All updted from outside the premises on smart phones. I dont know why more firms dont do this

The routine fire drill worked well and the local coffee and sandwich shops loved us on fire drill day!!! (the drill would be timed to avoid pub opening times)