Well, a couple of things to consider I think:
1) Why does perfect even have to mean that you conceive each and every time (unless you're already pregnant, of course)? Could be very good and natural reasons NOT to conceive easily. Meaning then that there is no way to know how much or little time they spent inside the garden.
2) Perhaps they did not 'lay together' until after they were given their garments of skin and cast out of the garden? (which is what I think you're implying, that Satan 'got' to her quickly.
3) Perhaps the story, while relaying a truth, is not about a 'flesh and blood' tree, apple, man or woman. Perhaps God did not literally walk on his feet in this garden, nor did Eve take a literal bite out of an apple that held all the knowledge of good and evil. But rather that all these things are being explained in a physical 'flesh and blood' sense, so that people can understand the simple story and lesson, and eventually move on to understanding the spiritual aspects behind it?
Symbols, of the spiritual thing they represented.
But for arguments sake, taking it all literally, as in flesh and blood man and woman, garden and snake, etc... then think about what you would do if you were just opening your eyes from nothing to an adult body. In less than a month, instead of being curious and overwhelmed by life itself and everything around you; all that you are seeing and experiencing for the first time... would you take one look at Adam, and then be comfortable 'getting busy' so quickly?
What would have been the hurry in letting Eve just learn for a while? They had no time-limits, no death, just eternity. Why would time have meant anything at all?
Tammy