Does anyone know the appropriate amount of tips to be given to porters and sherpas for a 4 week trekking and climbing trip. there are 3 of us on the trip. any ideas?

A general rule of thumb is for all trekkers to tip the guide. I am not sure what percentage is customary for a guide. Each trekker should tip his/her own porter about 15% to 20% of the wages he earned during the trip for good service. If you have any piece of gear/clothing that you do not like, it can be used as a tip in addition to the 15% to 20% cash.
If you went on an all inclusive trek in Nepal where you do not know how much the guides/porters make, a rough estimate of daily wages for a porter is about 600 rupees and a guide is about 850 rupees.