As said, it really depends. I recently issued a ticket to Dubai with MNL transit. For the outbound flight, I managed to find a pair of flights that allows me to have 23:30hr of transit time, which I can make a full one-day stopover at MNL without involving additional ticket cost and tax. On the way back, if I involve a stopover, I need to pay an addition $120. So you have to trial and error to work out the best itinerary. If it happens that your itinerary involves a stopover (>24 hrs) but for the same ticket price (total price will likely be a bit higher due to additional tax if there is so), then it is a free stopover. No hard rules as different city pairs and time of travel will yield different outcome