The 3-month salary rule is an often-cited guideline for determining how much one should spend on an engagement ring, suggesting that the buyer should allocate roughly three months' worth of their salary towards this purchase. However, it's important to note that this rule is not universally applicable or set in stone - individuals should consider their own financial situation and priorities when making decisions about purchasing an engagement ring as there are many other factors to consider beyond just adhering to this traditional guideline.