Re: HoW many hours should I practice daily?
Khaled, I'm with Peter on this - other than weapon work I'd really just suggest other forms of exercise, ideally both aerobic (for general health) and anaerobic (more needed for randori and sparring).
how can I practice alone? difficult
Is imagination enough? - can be, but the random element of a partner is missing so you can be misled on what actually works
should I practice daily?
and how many hours should I spend practicing? - to both I'd suggest do as much as you find interesting to do, if it's becoming a chore, leave off, stick with dojo practice and supplement with some other activity instead. You've got loads of time to learn aikido, so just enjoy the learning process and train as you see fit.