I think the manual said 250 ma not 250 mah.  If that is so take the numbers I gave you for how long the batteries below will last and multiply them by 3. 
Read that thread you referenced below all the way to the end.  Ya they are a little confused about how long the battery should last and I think that is due to the .75A  which I hope is incorrect.  They mention a battery form Hobbyking.  I own one of those batteries and it is a 4 cell.  If your detector will work down to 10 volts (Look in the manual) I would recommend a 3 cell battery. They also talk about fire.  The fires I have heard of were caused by one of 3 things.  Over charging (protection card prevents this).  Too high of current (protection card prevents this unless the current rise is fast enough to destroy the card before it can interrupt the current (then a fuse will work)). Mechanical damage to the battery(don’t stick your knife into the battery). The battery I suggested below is a grade B batteries (will not supply high(5+ A) of current) which is not stocked in US but there are some grade A batteries that are stocked in US.
Can I borrow your detector tomorrow I am going to go measure the current at different depths.