It's different for every sewer and every machine (and depends also on fabric and thread).
Basically what you have to do is test until you get it right. Cut 2.5" strips of fabric (maybe 6" long). Sew 3 of the pieces together with what you think is a scant 1/4" seam. Iron, then measure. With perfect 1/4" seams, the piece should measure 6.5". If it's less than that, then you need to reduce your seam width and try again.
Here's what I do to achieve perfection (ahem.....). I take one of my cutting rulers and place it under the presser foot of my machine. I lower the needle until it touches the 1/4" line, but then move the ruler slightly so the needle touches just a little to the side of the ruler's 1/4" line. I lower the presser foot to hold the ruler in place. I also check to make sure that the ruler is straight from front to back. Then I take a strip of moleskin (that I have already rotary cut), peel off the paper backing, then stick it to the base of machine right next to the ruler's edge. (Moleskin is sold in the foot section of a pharmacy. It is a thick adhesive-backed material used to protect feet blisters.) The moleskin provides me with a physical guide for feeding my strips into the machine so I don't have to stare at a mark.
At this point, you should test with the 2.5" strips to make sure the end result is accurate.
Instead of a ruler, you can use graph paper that has 4 squares to the inch.
The reason for the "scant" seam allowance is so the end result will be the correct measurement. Seams require some turn-of-the-cloth that uses up a little bit of the fabric. If the seam is an exact 1/4", then the end measurements after pressing and many seams will be short. Also if you are using thick fabric and/or thick thread, the turn-of-the-cloth will take up even more of the fabric. Better to take a little off each seam than to end up with a block that is 1/4" short from all the seams in it.