Nowadays the length of an engagement is determined by how long the couple needs in order to make the necessary preparations before the wedding. It also depends a lot on the persons involved. For those living with parents, a few months may seem an eternity, whilst couples already living together tend to take it easy.
The most obvious reason that delays a wedding is finance. Most couples prefer to go into their new home immediately after the wedding. This means that the house has to be ready furnished and equipped with all the minor details way before the wedding date.
Other couples might prefer to finish their studies before getting married, some might have other family weddings and so prefer to distance them a bit. Others prefer getting married on a certain date, so would prefer to wait and have the wedding they want rather than being ‘forced’ to marry earlier.
All this might create unnecessary tension, but on the other hand the engagement stage is a period in life when you can enjoy being together without the responsibilities of marriage.
So use your engagement for its real purpose: to get to know each other really well before going on to another stage in life.