You should be able to notice the effects of stimulant medications within a matter of hours. IR ritalin 10mg should work within 20 minutes to an hour. But wait a week or two to see if you notice any difference. I don't know how severe your concentration is so you might not notice it.

For some people it just doesn't work. Methylpenidate (ritalin) is usually used on people that are hyperactive more than inattentive.

If you have no luck then you should try a different medication. Don't try strattera next because it takes too long and doesn't work as well as stimulants for most people. Something like Adderall would be good or dexedrine, vyvanse, etc..