How many action potentials from presynaptic neurons would be required to make a postsynaptic neuron fire? I am looking for a rough estimation of the number of action potentials from other neurons required to cause a neuron to fire?

I read here that a potential of ~ -55mv must be reached before an action potential is fired in a neuron, my thought is how many action potentials from other cells would (roughly) be required to reach this - and in what required time (if it matters)?

I am not a neuroscientist and suspect this question is unanswerable but though this was the place to ask!
It is very very important to note that in the brain, most neurons are receiving input from way more excitatory synapses than necessary to bring the neuron to threshold.  The thing is that they are also receiving input from a huge number of inhibitory synapses as well.  This means that it is not how many inputs that are active at a given time that determines whether a neuron fires or not, it is the balance between excitation and inhibition.  

see:
attalah and scanziani 2009 (experiment) http://labs.biology.ucsd.edu/scanziani/documents/AtallahNeuron09.pdf

okun and lampl 2008 (experiment)
http://www.nature.com/neuro/journal/v11/n5/full/nn.2105.html

van vreeswijk and sompolinsky (theory)
http://www.sciencemag.org/content/274/5293/1724.short