We were engaged for 15 months but would rather it had been a lot less. We had known each other for 10 months when we got engaged (and were 'romantically involved' for only five of them
). The length of our engagement was pretty much dictated by the fact that Mrs T (then Miss G) had committed herself to a year as a volunteer schools and youthwork organiser with Scripture Union Scotland after she graduated.
We never lived together before we were married, although she and I both stayed at her parents' for the few weeks before the wedding, being otherwise homeless at the time! (We bought our house immedaitely before the wedding and moved in immediately after honeymoon).
As for your engagement ring, the first thing to say is avoid Elizabeth Duke at Argos like the plague. It is cheapo cr@ppola. They say you should spend a month's salary on a ring, which I did - about £750 at the time (local newspaper hacks are not well paid). Someone mentioned the 'four Cs' above, that is good advice. If you get your ring from a reputable high street chain like Beaverbrooks they should give you a certificate with the full details of your diamond on it, so you have some idea of its quality. It is unlikely you will be able to make an educated judgement yourself about the exact quality of your diamond, unless it is especially poor or if you have one of different quality to compare it with.
All that said, Mrs T says if I had proposed with an Elizabeth Duke ring she would still have said yes (or a ring-pull for that matter). After all, it's about a lifetime together, not a piece of jewellery.