How long? far too dificult to answer with any degree of certainty. Water absorption has nothing to do with crazing, you can put of piece of extruded acrylic in a bucket of water for years with no crazing. Crazing is caused by stress and nothing else. Using material that is too thin for a given application will stress the material and because extruded has limited resistance to stress - will craze much sooner and much worse than cell cast. Solvents that glue the acrylic also stress extruded materials and the application of Weld-on 16 or triangular rod afterwards can cause crazing just as easily as alcohol which of course is part of the chemical make-up of acrylic solvent.
Make your tank but please use at least a perimeter flange (aka eurobrace), do not flame polish *any* of the material, and don't use any alcohols on your tank post gluing. Follow this and I can just about promise that you will not have any crazing issues. With regard to what size bracing; try a 1-1.5" eurobrace with a 3" crossbrace so you'll have 2 openings measuring roughly 12 x 7". If you are gluing baffles into the tank, you can forget the crossbrace. If the tank was say 18" front to back, you could simply use a wider perimeter flange and forget the crosbracing aspect entirely but because the tank is only 10", your opening would be too small. The bracing will limit deflection and stress on everything from the panes to the joints so there should be no issues for quite a while if ever.
HTH,
James
