Imo, you'd end up having to supplement the pleco with other foods as they require more than just algae in general. A good choice would be a couple nerite snails, you don't have to supplement their diets and they only reproduce in brackish so they won't take over your tank.
It also depends on the type of algae. Not all types of algae are palatable. Some need manual removal no matter what you have in the tank. And yes plecos typically only eat algae while they are growing. Once they reach adulthood, they will tend to resort to algae if no other option is available.
5 to 10 years
well i think the pleco i have in my 20 gallon tank would love to eat my plants considering he eats lettuce an idk what kind of algae it is u can see it when u look through the side of the tank the glass looks fuzzy kinda