It definitely depends on the relationship. If it was just infatuation rather than love, you'll know when you've got past the 'honeymoon' stage because you'll start to see their flaws and everything about them will annoy you.
When my ex and I met, it was quite intense. We were an item within a fortnight, and spending as much time as we could together (we lived in the same building). Fast forward about 6-7 months and there were signs that he was getting fed up of it, and didn't think we should be spending as much time together, whilst I'd become dependent on him for happiness and tried to spend as much time with him as possible. Looking back, I wasn't in a good place mentally, but it caused a lot of arguments - especially when we moved out of halls and into separate houses - and actually throughout our four and a half year relationship, even after I got help for my anxiety & depression, how much time we should be spending together was one thing that we argued about a lot. Towards the end we'd been long distance for a while and I'd had the time away from him to reflect on our relationship and realised how unhappy I was, and how unhealthy a lot of it had been for me. I don't regret it at all, as there were times when I felt on-top-of-the-world happy and we had fun together, and I've learned a lot from it, especially about what I don't want and what I'm willing to put up with from my partner.
I think an intense start can work as long as it's a healthy relationship, as long as you're still doing your own thing sometimes too and it's not stopping you from seeing your family and friends and pursuing your other interests, and one of you doesn't become dependent on the other for happiness. There's a difference between a relationship that moves fast and is healthy and one that becomes all-consuming and as long as it's not the latter like mine was for a time, there's no reason why things won't work out. I know in the future I'll be very careful not to let a relationship take over my life so quickly and completely, but then I've grown up a lot since I met my ex.