Typically I aim for 1 to 3 fights per session (about 4 hrs). It's often good to start with some light action, nothing too crunchy, but something that wakes the players up and also provides a plot twist. Usually unarmed combat works well here, or escaping villains, etc.
Then a small combat scene mid-session works well for my troupe, although it depends on the plot etc, but I usually tend to design the main combat scene later in the session, typically the climatic scene works best.
RQ combat can be deadly, so any more combat scenes then this could be big trouble for the characters. If the mid-session combat is pretty crunchy, then you often need to provide opportunities for healing, so I sometimes allow for a week or two to elapse. If the plot doesn't allow for this, then provide some opportunity for magical healing, otherwise the characters will have major issues entering a major combat scene at the end of the session.
The fact that combat is deadly for even experienced PCs is one of the best things about RQ, but an ex-D&D GM needs to be aware of this and not stack too many odds against the characters.
Have fun