You need to find that out from your bike's frame manufacturer. For example, Specialized frames have a tiny hole which marks the minimum insertion depth of the seatpost. If there is no mark on the frame, get ahold of your bike's frame manufacturer. Don't "guess", you could break your frame....
Whatever you do, don't assume the minimum insertion for the seatpost is the same as the minimum insertion requirement for the frame. A lot of seatposts only need 3" of insertion to prevent the post from bending, but most frames require the bottom of the seatpost extend below the lowest point of the top tube/seat tube junction to avoid frame damage.
Yes, it is marked on the seatpost, however the true "weight weenie" is going to cut the seatpost to it's absolute minimum legnth to shave as much weight as possible. That's why using the frame's manufacturers minumum insertion legnth is most important in this application.
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