I usually add sugar based on quantity in a 5.5 gallon batch...
- Anything over 1 lb simple sugar in 5.5 gallons, make a syrup & add when fermentation slows, probably around day 3
- Anything under 1 lb simple sugar in 5.5 gallons, add with 15 minutes left in boil
Adding to what beergolf said...if you use too much simple sugar in the boil, the yeast will eat those sugars first during fermentation. Then, it's possible they'll "get full" on those, slow down, & drop out before they consume the complex sugars from the mash/grain leaving you with a sweeter beer than you anticipated. For large quantities of simple sugar, making the syrup & adding when fermentation slows will help dry out the beer & ensure the complex sugars from the mash/grain are consumed first.