How long is a piece of string? 
There are so many variables here, people can give you averages, but like all averages that doesn't say anything about you and your particular relationship.
A lot depends on your sex drive and your own state of mind, as much as it does on any physical connection between the two of you.
But on average, the honeymoon period, in which the brain is producing lots of lust-inducing hormones, lasts between 6 months and 2 years, with 12-18 months being the most common. After that, common wisdom is that the hormone balance swings towards those that promote longer-term bonding.
But you can effect the timescale enormously by how often you're having sex, how often you see each other, how well you know each other, how 'sated' you feel after sex, external factors such as work/family/your own priorities, etc.
A lot depends on whether you want a good relationship with lots of good sex or just lots of good sex. Either of which is fine. If you want the latter, don't get too personal. There's nothing like knowing your partner's intimate habits to lose a sexual thrill. In a relationship the benefits cancel that negative, but in a sex partner it just kills desire.
IMO, a really good sexual connection withstands the test of time. Few people can really maintain that crazed state of the initial honeymoon period because most of us would drop down dead from the stress that hormonal cascade places on your vital organs and nervous system. However, that doesn't mean it disappears, it just mutates.
All the couples I know who have stood the test of time and are happy, with good sex lives, still have that sexual connection - it's just not as overpowering. They tap into it when they want sex rather than it telling them they want sex every minute they're together IYSWIM. For couples like that, the act of being together and enjoying each other's company is what is craved, rather than sex, and it is this level of intimacy which can hold couples together. If you want a LTR this is the holy grail I suppose. Ironically, to maintain that, what you really need is to be really happy with yourself and your own life first and your partner's attractiveness second. It's easy to feel turned on if you feel desirable.
But for now? Who cares! Enjoy!