Assuming a sharp full sized fine cut file, with a good handle, (and not made of Chinesium), and the work rigidly clamped in a bench vice, you should be able to take off most of that 0.7mm in a few minutes each. Note this assumes ordinary mild steel or medium carbon steel bolts, not above bolt grade 5, and not specially hardened. Any alloy steel of high hardness or with a tendency to work harden would be *MUCH* slower.
A bigger problem is getting the filed down ends uniform and square. Measure often, and change your tactics when you've taken off 0.6mm on the low side. Mark the low spots with a marker pen then file the rest down to match, again checking the measurements frequently to avoid going too far. Ideally you should have around 0.05mm left to remove over the whole end of the bolt. Then you need to mount the bolts to a convenient sized flat plate in a square, and lap their ends all together to get them square and down to final length. Wet lapping with ascending number grits of Wet & Dry paper face up on a hard very flat surface (e.g. float glass on some sort of backing board) is probably the best way to go here. Again measure often and once it stops rocking, concentrate pressure on the highest corner or side, to take those bolts down faster to get them level. Finally, remove them from the plate and 'break' the edges or chamfer the last turn of the thread.