It depends entirely on how much memory you have. I've been learning about this a bit over the past few weeks with some issues I had. There is a rough formula which someone will be able to expand on and give you something far more accurate than me as I don't fully understand bits and bytes yet. But take your array and work out the size. You can do this with object.size(x), or very roughly by number of rows by number of columns.
I had a data.frame with 2 million rows and 250 columns, and an apply statement took 20 minutes to complete on my dual core 8gb ram macbook. When I moved it to our 24 node 32gb ram serve it completed in 15 seconds.
I think as a rule of thumb (happy to be correct), if object.size is anywhere near 75% or more of your ram you will have computational issues and hanging, if you're at 120% of your ram then operations might run for hours depending on what they are.