First batch: I'm ready to botte a Cream Stout... Priming Sugar?

I'm ready to bottle my first batch of beer. It's a Cream Stout recipe I got from the supplier. The recipe says to add priming sugar to the bottling bucket before I transfer the beer from the carboy. But, it doesn't say what kind of priming sugar or how much and the only sugar that came with the recipe was 1 lb of maltodextrin which was added just before I finished the boil, in accordance with the recipe.

I have about 4.5 gallons of beer in the carboy. The beer is very slightly sweet, flat, and has a really bitter aftertaste. Any idea how much and what kind of priming sugar I should use?

I use corn sugar, 3/4 cup per 5 gallon batch. I boil it for about 10 minutes in 2 cups of water and give it an ice bath in the sink until it comes down to under 80 degrees, pour it into the bottling bucket and rack the beer on top of it.

The real of thumb for priming is about 1 oz. of corn sugar per gallon of beer. Disolve in some boiling water. Let it cool. Poor and rack beer on top of it.

Cool. I hope to have time to bottle it this afternoon, definitely by tomorrow evening. Is there a time limit in which the beer has to be bottled before it spoils?