Any rule of thumb would be dependent on the physical build of the boom, and I doubt that is standardized enough to give a generic guideline. However, as an R&D engineer who also does photography, I can comment on the mechanics, which might help ...

It comes down to the length of boom you want (from stand to lighting head). As you know, the longer this lever arm is, the heavier your counter weight has to be, by the equation L1·W1=L2·W2. (But the boom sees the force of the two masses, W1 + W2.)

If you seldom or never care much about the boom length of your lighting kit, and you're worried about the materials bending too easily, then a good choice to lighten the load on your boom will come from placing the boom at it's lengthwise midpoint on the stand, and using a counter weight equal in mass to the weight of the lighting head.

An even shorter boom (like 1/3 stand to lights, and 2/3 stand to counter weight) would allow an even lighter counterweight, but you would surely be into diminishing returns -- that is, the boom had better hold up just fine under twice the weight of the lighting head, so going any lighter-still gives you no further structural benefit. Conversely, a longer boom (e.g., 2/3 stand to lights, 1/3 stand to counter-weights) would require a heavier counterweight (twice as heavy in this example), increasing the net load working toward bending the boom.

Still, unless the boom pole is getting close to 4 mil aluminum (like a pop can), it should be okay at a 2:1 position, though. Even a 4:1 position should not be too gross a load (at five times the mass of the lighting head: 1 for the lighting head + 4 for the counterweight).

Trust your instincts. I bet you have gut feel of what's too extreme a boom reach. Say your lighting head weighs 5 lbs. With the boom at a reach of 9:1 (90% of the boom on the light side, and 10% on the counterweight side), the counterweights will weigh 45 lbs, and the net force at the stand, on the boom pole is 50 lbs ... which probably would strike anyone as excessive.