It's a market question. Is there a market for your ideas? Is your time in demand? Then you can charge what the market will support.
But it's also a social question, one of balance. Even if your ideas have a good market value, where's the harm in a little free advice? The harm is only in an unreasonable amount of free advice. If you're a smart person whose ideas mean a lot to others, it's only being a good neighbor to offer a quick opinion on a few things if you aren't busy. But by the same token, a good neighbor only asks within reason.
But it's also a social question, one of balance. Even if your ideas have a good market value, where's the harm in a little free advice? The harm is only in an unreasonable amount of free advice. If you're a smart person whose ideas mean a lot to others, it's only being a good neighbor to offer a quick opinion on a few things if you aren't busy. But by the same token, a good neighbor only asks within reason.