'unreasonably long' is correct and usable in written English.
You can use it to describe something that is longer than what would be considered reasonable. Example: The manner in which my uncle told his story was unreasonably long.
That is an unreasonably long interval.
The screening and security checks, she said, are unreasonably long.
Three years is an unreasonably long time in a child's life.
The silences I've learned to let hang in a classroom seemed unreasonably long.
Both had challenged their sentences as unreasonably long compared with what others have received.
I expect to work hard but don't want a job that takes over my life with unreasonably long working hours.
After the oil-processing tax was defeated in the legislature, Long — not unreasonably — accused lawmakers of having been "bought".
However, Facebook launched a new Safety Center in April and its long argued – not unreasonably – that panic buttons imply that social networking is inherently dangerous, which would be a warped way of looking at things.
A study comparing long-term observed, LOCF and NRI efficacy data clearly demonstrated the conservative nature of NRI, 21 with a further report suggesting that LOCF can lead to unreasonably high estimates of success in long-term analyses, depending on the reasons for patient withdrawal.
But showing more leeway, Mr. Balter wrote that interns could do occasional work done by regular employees, as long as it "does not unreasonably replace or impede the education objective for the intern and effectively displace regular workers".
As set out by the judge in that hearing, the case law means that there is a public right of peaceful assembly on the highway, so long as it does not unreasonably impede people passing.
