Depending on his age, health and other factors, I'd say average would be from less than a minute to a few minutes, if you are actively trying to gain an erection. If you are talking about a passive erection with no stimulation then it can take a long time.
If the nerves are damaged it may not be possible to get an erection naturally
about 5-6 seconds if you are real sexy
You will have to see a sexologist and follow him religosly.
Apart from obesity or malnourishment food have very little impact on erection. There are medications that can have that effect, but I doubt you'd get them prescribed.
Achieve what ? Ask this question a little bit detailed
erection
It depends on the size of your kitchen and what kind of design you want to achieve. If you want to get a simple design then it will take 3-4days. If you want to get a modern designed kitchen then it is recommended to hire a professional and it can take a fortnight.
a short term goal doesn't take as long to achieve
It can take up to one year after prostate surgery to know if you will be able to achieve an erection. Also, if the nerves that run along the sides of the prostate gland, which control erection, were damaged or cut, this could effect the ability to achieve an erection. See your doctor for a follow-up exam to see if the nerves were in fact cut or damaged, or if your recovery is still taking place and it may take a bit longer for normal sexual function to be restored. I would recommend also consulting with a doctor who specializes in prostate surgery and rehabilitation to regain sexual function. If you are having difficulty achieving an erection, a specialist may have tools and therapies available to help restore this function for you.
As long as you allow it. Take Ibuprofin.
No
it depends on your average speed and the type of ground you are walking over. For example on a flat even surface you may achieve about 4mph, in which case 90 miles will take just over 22 hours.