You'll also have to give a number of keyboards built over that 18 months too... I've had a 1lb spool of solder for 25 years, doesnt mean squat.
You could probably figure out an exact number if you took the following:
1. solder a pad and measure how long of a piece you needed to make a good joint
2. cut another piece off that is that size and weight it on a postal/kitchen scale. (too light? multiply your length, weigh, and then divide)
3. now you know how much each solder joint weighs... multiple by number of joints (2 per switch).
4. now you know how much each keyboard costs in weight from that 1lb spool... divide and win
So just a quick guesstimate would be i'm going with say 4" of solder per joint, guessing that weight (lead is heavy) at 1gram (paperclip weight). Google tells me at 208 joints/grams (full size 104 keyboard, no LEDs) thats .458 lbs.... so I would get 2 fullsized keyboards from a 1 lb spool of .031