The mileage may vary depending on the condition of roads that you drive on and how slow/fast you drive over speed bumps, pretty much depends on how worn down they are. One way, not the most accurate, to look at the condition of your struts is to push down on the front of the vehicle and see how the vehicle reacts. If it goes back up and bounces a lot then your struts are worn, and if the struts go back to its regular ride level with littler to no bouncing then your struts are good/fine. If my description was not great just look up a youtube vid on checking strut condition.
I have an 09 ex35 with 121k miles and the any of the struts have not been changed and my front struts are a bit worn but I know I can wait a bit longer before changing them. To my knowledge I have only seen KYB or Monroe struts for aftermarket but I do not know if they are made to replicate OEM ride. It is tough to find aftermarket parts for this car which is for sure a downside for this vehicle. For example my intake boots were cracking so I looked at oem replacements and they were cheapest at $180 per boot, so I did a little research and found that the mishimoto aftermarket boots for G35 and 350Z, $130 for both boots, fit but need a little cutting.