How to stay concentrated for a long period of time?

I think the answer to your question is "people don't concentrate for 2 hours at a stretch". That's why the pomodoro technique is so useful!

I'm very focused in general, and I find 45 minutes to an hour is the longest I can sustain my attention on a task, especially if it's boring. Contrary to the other poster I don't think that I have ADHD (though I don't doubt I'd be able to focus for 3 or 4 hours straight if I took dexies, since my husband *does* have ADHD and the medicine does that for him).

You should look online for resources about combating procrastination, getting motivated, etc, as it's the same sort of problem everyone runs into.

Here's my personal productivity tips:

• Pomodoros: I use the complice.co less wrong study hall, which has 32/8 pomos. I like that balance as 32 minutes is coming on the maximum amount of time I can be "mostly reliably" focused. I won't bother explaining pomodoros as they're on about ten thousand different websites.
• Beemindier: I've been using beeminder since its official launch and it's got me to do everything from an 1800 day duolingo streak to reading a textbook to writing a 50,000 word novel to studying several hundred anki cards a day. If I don't, I have to pay $5, and I'm so cheap I'm not ever going to give up the $5.
• I reward / bribe myself. "Can't start making breakfast until I've done one pomo", "that pepsi you're craving will only be yours if you've done a pomo of project status reports", etc. Only ever small things that I get immediately after my equally small achievement.
• Don't be too hard on yourself. The biggest thing that gets me in my work day in terms of productivity is my "shame spiral". I don't get much done which means I feel shitty about myself and keep on not getting much done, "the day is wasted anyway". I try to think what my best friend would say to me and internalise that rather than thinking what a failure I am. Also, if I have a real shame spiral sort of day, I try to change my to do list to just one or two stupidly easy things, or break tasks down to steps the include "open the program", "click on the link", "type in the name of what i'm searching for". It's stupid, but it's so motivating to give yourself some wins.

I am bad in concentrating, but good in getting things done to a deadline. So I think that concentration itself is not so important, if you can do the work. I am distracting every 5 minutes to check facebook, news etc. I interpret this as that my "pomodoros" are very short and I just need to rest every 5 minutes.

Note that people with ADHD often have hyperfocus, and can concentrate on certain things for much longer than two hours (e.g. videogames), they just have poor attentional control.