I think a timeframe may be more suitable in an environment of steady house price growth, however with the market the way it is in Ireland at the mo, I'd also be inclined to set a benchmark price where the two of you sit down and have a good chat about the option of selling up.
I'm assuming you bought into the market together so that you wouldn't be priced out of it a few years down the line. With that in mind, making a decent profit ranks up there with having a home to live in - a different situation to a couple buying together, for example. Assuming you'll each buy seperately after this house, your minimum profit on the sale will need to cover the estimated stamp duty costs of buying 2 seperate homes and returning the deposits you both put into the house, and that's just in order to effectively break even. Beyond that, I'd be hoping to make enough to cover two seperate deposits at least.
You should also bear in mind that your risk is somewhat different to a FTB couple. As I said, the FTB couple would only be buying one house if they were moving on, whereas you guys would probably be buying two seperately, with all the stamp duty and other transaction charges that entails. Secondly, the FTB couple could always stay put if the market goes pear-shaped (assuming they stay together!) whereas if one of you meets the love of your life, you could be in a right pickle trying to move on after a market slump/crash/soft-landing etc.