Two one-ways or round-trip?
 

I'll be flying from Philadelphia to Kuala Lumpur in May 2018 and flying back from Kuala Lumpur to Philadelphia in January 2019. My question is: Would be it cheaper to purchase PHL-KUL now and then wait to purchase KUL-PHL? Or would it be cheaper to purchase round trip in February 2018, bearing in mind that I would fly to Kuala Lumpur 3 months later?
Thanks!

1 way international is almost always more expensive than RT. You can price it out now on Google Flight and see how prices fluctuates.

There are far too many variables for anyone to predict which way might work out cheaper. I have flown quite a few times with long terms between flights such as you describe. What I did was buy return tickets that allowed me to change the return for little or no penalty in cost. When something is that far in advance, I always want to make sure I can change the return if necessary.

I don't try to find the cheapest fare. I recognize that given the way airlines price tickets in real time these days using 'dynamic pricing', it is impossible to predict when to buy to get the lowest fare. It doesn't matter if it is a one way or a return in 2 weeks, never mind months apart. I do some research to try and determine the price range the route I want to travel tends to sell for. If the cost of two one-ways or one return varies between say $1000 and $3000, then I decide how much I am willing to pay. If I decide I am willing to pay $1500, then I look for a booking at that price. Once I find one, I book regardless of how far in advance and regardless of whether it is one way tickets or returns.

I normally find that booking the return trip is cheaper than booking the two one way tickets.

Given your handle on this forum, you do realise that a 'return' trip on budget airlines is usually the sum of two one-way tickets? ;)

What class of service? If you are flying Coach, then the difference could be $; if flying Business, then the difference could be $$$.

Also, have you considered flying from one of the DC airports instead of Philly? It is usually cheaper to fly from "WAS".

Good point farci, many low cost carriers these days price all their flights as one ways and a return as you say is simple the total of the two. There is still no way to know when you will find the lowest price though and it is important to make sure you can change the return date when looking at a time so far in advance.