Taking into consideration the natural stress that each word has, we can easily identify the stress patter in this line. First, we start with those words which have more than one syllable, and identify the stress pattern within them.
FORty WINters beSIEGE
In the context of the sentence, they look like this:
when FORty WINters shall beSIEGE by you.
The words forty winters are the clue here as they establish a pattern of stressed+unstressed that can be used for the rest of the sentence.
