The period for which Levodopa etc is effective

I am not on medication yet but have read that Levodopa for example is only effective for a limited period. My question is, if in my case , its effective period is, say, 5 years, but I start it in four years time, can I still expect/hope to get 5 years use out of it or is it more likely to be 1 year (ie 5 minus 4). Or is it not that simple? Hope that's clear!

I am getting conflicting advice. A neurologist I saw this week is firmly in the 5 minus 4 camp. I have however read the opposite.
This seems to be the current mode of thinking.
I`ve taken these quotes from two reputable PD websites.
Hope it helps.

There is no evidence that delaying therapy results in longer benefit from levodopa. The prevalent myth that levodopa works for five years and then stops working is just that–a myth.

Levodopa stops working after five years. This is
perhaps the single most pervasive myth about PD
treatment. Many people are reluctant to start taking
levodopa because of fear of “using it up.” Some
physicians also share this “levodopa phobia.”
Levodopa works for decades. Levodopa does not
treat all of the symptoms of PD, but it dramatically
helps many of the most disabling motor symptoms.
Thank you, Jacko. I understand it better now.