i like to trip semi-regularly for short periods. say, 3-5 weekends in a row - to really immerse myself in psychedelic thought - then take a break of at least a month, usually more, depending on how busy i am at the time. sometimes 6 months will elapse between trips when the right opportunity doesn't present itself, but it's always great to come back to it fresh.
yes, you can overdo psychedelics - the "magic" may fade in frequent trips.
i always find the first of a series of trips has the most striking effects, and subsequent trips are less crazy, but this comes back with a break.
@BottleOfOxy - yep, time and absence. the tolerance is something that (in my experience) comes back with no effort - just rest and recharge, try and integrate the psychedelic experience into 'normal' daily existence. i've always known when it was time to have a break; the unpleasant effects become more noticeable, the 'amazing' elements of the trip lessen. not so much awe, not such profound feelings.
there may be a more long-term tolerance at play (i tend to take twice as much as novice trippers to achieve similar effects) but i put this down more to familiarity with the drug and my desire to have an intense experience. what is foreign and confusing to first timers is something i am very comfortable with, so i dose higher. i've still had great nights on single (or even half) hits, so it's not like permanent tolerance, just a dosage preference.
the best trips come after a break of a month or more IMO, but there is nothing wrong with tripping weekly. i know that when i was tripping as often as my tolerance would allow - about twice a week - it stopped being as fun, but i certainly got to know the substance (LSD) and found some value in immersing myself in such an interesting chemical.
i wouldn't recommend weekly dosing as a long-term practice, but 3 weeks in a row is not something i'd be concerned about.