It's an instantaneous effect with no save, so it is in effect forever. I am currently effected by this exact case, only I made the mistake of trying to use the rod a second time and low and behold I am now 144th my original size, and since then, I have done a lot of research on this particular effect. Also, it should be noted that this spell does NOT act like the reduce person spell as some would argue, otherwise it would say so. Thus... you have to find and use the statistics, penalties, and bonuses that apply to a Fine sized creature and apply them to your character. I believe it is -10 Str (carrying capacity 1/6), +16 Hide, +8 to AC and Attack, and -16 to Grapple. Also... walking can only be reduced to 5' as a full round action. Thus, unless your character can fly, he has to be carried or live with taking a full round action to move a 5 foot step. On a final note, all magical items except for armor and weapons resize, even to fine sized, and when calculating weights, factor that at 1/12 for items also. Good Luck by the way, as a 1/2" spellcaster I know I am having fun.