10% is sufficient each week. RODI is not really negotiable, depends on the TDS out of your tap really but best to have a reading of 0 TDS to control algae outbreaks etc. Some people use pure dew bottled water. Re how long should a bucket of salt last - thats like asking how long is a piece of string. It depends on the salinity level you are wanting in your tank and the type of salt you use as not all salt is created equal. You will also need a refractometer to check the salinity level of the water you mix as it is rare that following the directions on the label you will get exactly what it says.
I made up artificial salt water using RODI water and buckets of salt. I also went down to the Takapuna Boat Ramp and collected natural salt water, ran it through a 100 micron filter to get rid of nasties, diluted it a bit to get the salinity level suitable for my tank and used that. You can do this if you go within half an hour each side of high tide on days when there hasn't been rain for at least 24hrs and the swell is not too heavy. Sea water has salinity of 1.027, most marine tanks run at 1.025.