How long do blessings last? I recently attended a Tibetan Drinking Kagyu Buddhist retreat. During that time the visiting Rinpoche blessed several items, and I was told by His Eminence that one can renew the blessing of statues by placing them in a small bowl containing a little bit of saffron water, and then dribbling some of the water over the top of the statue. I neglected to ask how often this should be done. Does anyone know?
I went to a famous temple in Rosemead and some ancient sacred items (sariras supposedly) were brought out and I blessed my amulet on them. I left the temple literally half an hour after that and on my drive back home had an accient and my car burned down, totalled. :-(

To answer your question, karma and merit is a complicated topic to understand and I don't think anyone will be able to give you a precise answer. One would have to go into the origins of said karma that created this object and those are either infinite as some sutras suggest, incomprehensible (as Buddha avoided the questions about origins of karma), or nil (see Zen master's response to king).

You will have to feel the object yourself. Try feeling with your non-dominant hand which is usually better at perceiving these energies.

This whole enchanting thing and sariras is something that comes from Vajrayana and the newer traditions which the original branches of Buddhism did not advocate, espouse, or uphold.

As much as I believe in crystal energetics and various holistic technologies, I am now wary of "sariras" and know that the highest way to make merit is through appreciation of the Buddha, his teachings, and practice whether it is in the form of another object, ourselves, or others. Try practicing different mantras--THAT is definitely IMO way more powerful than any sarira.