future tense

[alikim]

Hi,

My question is:

I'm in the middle of doing something, I already have spent two hours doing it, and I need three more hours to finish, so the whole job will take five hours.

What's the right way to say it:

"I will spend five hours on it when I finish"

or

"I will have spent five hours when I finish"

or something else ?

Thanks!

[allenman]

Your suggested reply sounds good. I would say
"I will have spent five hours by the time I finish"

Not a teacher -- AmE native

[alikim]

Thanks!

[emsr2d2]

There's no single absolute right answer to this.

I'll have been doing this for five hours by the time I finish it.
I'll have spent five hours on it by the time I finish.
It will have taken me five hours by the time it's done.
By the time it's finished, it will have taken me five hours.
By the time I've finished it, I'll have spent five hours on it.

We have various constructions for this but as you can see, in my opinion, "by the time" would be an integral part, rather than "when".