DH and I will be traveling Bucharest to Budapest in September. I’m trying to decide how far out to buy the tickets.

Any thoughts? I’ve never flown the airline before.

I think most experts generally suggest the best window between 30-120 days out. A family member travels to and from Cluj, Romania, regularly, and they don't see the wide fluctuations in fares that we see with US legacy carriers. They use other airlines. Right now, it's less than $100 pp. Although you never know, I would probably book no later than May or June so long as your travel plans are certain. We have other European flights with other discount carriers for the fall and have already booked them just for the peace of mind and to be done with it; and we doubt the prices can go much lower.

In the past, when I regularly check rates, it seems they invariably always rise with successive searches. And, that may be because "cookies" in my computer tell the airline computer that I have been checking. Look into deleting cookies and going into "incognito" mode when doing a subsequent search.

I don't think it is any extremally busy destination. There are some busier ones in neighbouring countries. So I think, general rules apply...

There isn't a general rule about when to book flights, when you see a price that you feel is good, book it & don't look back. Fares changing on successive searches is not down to cookies but down to inventory changes, some fares go down as well as up.