How many weeks does it take for claritin (loratadine) 10mg to work? how long should i give it a try before deciding to switch to a different antihistamine?

A few hours: Claritin (loratadine) should start working within a couple of hours. If it doesn't not help at all, then switching to another won't do you much good. If it does help somewhat, then you may try the other alternatives which may possess more antihistamine effect than Claritin (loratadine).

Try for a few days: Antihistamines begin to work in less than an hour but a person may not feel significant relief for a few days. Claritin tends to be one of the weaker non-drowsy antihistamines. Others available without a prescription are Zyrtec, Allegra and Xyzal. If antihistamines are still not working then an evaluation by an allergist can be helpful to determine the cause of symptoms