My personal liking is enough lemonade concentrate to make 1.5 gallons of lemonade for 5 gallons of beer. Most lemonade concentrate that I've come across is 1 can for 2 quarts, so you'd need 3 cans. And you skip the water addition for the concentrate.
Using homemade lemonade would be tough to calculate. But...if you have a homemade lemonade recipe already, I suppose you'd just take the necessary amount of lemon juice and sugar that you'd use for 1.5 gallons of lemonade and dump that into the keg then rack the beer onto that mixture. Purge the air and then shake well and then immediately chill it so the yeasts don't eat the sugar.