How long will it take for a very dead iPhone 5 to charge?

Need to charge my iPhone 5 up, it's been dead for probably about 3 years sitting in the box.

I have had it plugged in for about 35-40 minutes and it was showing no sign of life, totally black screen.

I saw something online that said if I charge it for about 30 minutes and then hold the home and lock buttons down at the same time it should re-boot.

I did that and it didn't re-boot, but it did flash up the battery image showing a very little bit of red at the bottom of the battery.

Does this just mean it needs longer to charge, or is it just totally dead and there's no hope?

Added (1). I decided to just leave it charging and after about an hour it buzzed to life!

All you can do is wait and see; it normally takes at least two hours on charge before it's worth trying the two button reset, in my experience.
Until it has rebooted and is working normally, the battery charge controller does not work the same and it can take a very long time to get even a partial charge.

Three years is a long time for a lithium battery to be left flat; they do eventually short out internally if not kept with some charge in them, and once that happens your only option is to fit a new battery.

When a lithium battery reaches a certain low level of charge, it starts to become irreparably damaged as its chemical structure breaks down, the copper inside it leeches into the electrolyte. When an attempt is then made to charge it, it forms a copper coating over the graphite anode, which is then further coated by a reactive coat of lithium metal. In a severe case, the coating will completely disrupt the charge cycle and so the battery will simply not charge. The cell can even start to bulge and there's a risk of fire. The only thing you can do is replace it.

(When the phone reports the battery at 0%, it isn't 'really' fully discharged, it has enough charge to avoid the above for some time, and a lockout chip prevents the phone from drawing power from it. But even a battery like this will 'deep discharge' given enough time. I think one year would do it, but it's virtually guaranteed after 3. )

Edit: Saw you got it working. I would replace it anyway as the cells are almost certainly damaged, hence how long it took for it to show signs of life.

You need to try a couple different charging cables and chargers. Also try to clean out your charging port with the end of a paperclip.

It may never charge because the battery was not kept up as it should have been.

Your phone's battery is permanently dead. Since it hadn't been charged for so long, it self-discharged to a point where it can no longer be charged again.

Gtased

If your iPhone 5 is completely dead, So firstly you should charge it for 30 minutes and weight, at that point hold the home and Sleep button in the meantime the telephone ought to reboot, except if the battery has bombed totally. Generally, It should take around 3 hours for a full charge utilizing the USB adopter.