I don't think it's unreasonable for a singer to accommodate the band with regard to a key change interval of a whole or half-step, or not. If your singer cannot do this, then maybe you need to find a better singer. But please read on.
The short answer is that it depends on the particular song paired with a particular singer, and there is no hard-and-fast rule about it. If a particular song contained a lot of notes that are all the way at the bottom or all the way at the top of a singer's range, then I can understand why a singer would want to change the key accordingly.
Most singers have a range of less than two octaves, depending on what they were born with and what techniques they have developed. The different voice types; bass, baritone, tenor, alto, mezzo and soprano, each have a different range of pitches in their natural range. A small number of women have a very wide range, from high tenor to soprano, but most do not. Some men learn to shift effortlessly from their natural range up to falsetto, but few can do this and make it sound musically useful.
The term to describe the range of notes that must be sung in a song is tessitura. It describes not only an "inventory" of all the pitches, but whether or not the melody lines are weighted towards the lower or higher range. This is different for every song, and there is no precise formula for measuring it.
To avoid disagreement, it's reasonable to ask your singer to work out what songs he or she would like to sing with your band in advance, and to agree before the jam session about the keys. If your band and the singer cannot agree on a key transposition on a certain song, then it just won't work, and you should ask the singer to suggest different songs that might work, and sort it out beforehand. Make it the singer's responsibility to provide his or her own written-out charts in the key that you agree on, if the members of your band cannot transpose a chart on sight for a difficult song.
And never ask a guitar band in standard tuning to play a song in the key of E-flat.
(The latest generation of iPad fake book chart software, such as iRealPro, automates the real-time transposition of charts like magic, but this product only provides for transposing the chord progression, since it does not support displaying the instrumental melody lines at all.)
If you get the idea that you are working with a singer with a weak voice and a very limited vocal range, who always asks for unreasonable transpositions, then perhaps you should look for a better singer with better technique. But if the singer is the star of the show and gets the gigs, then you may have to accommodate them.
In 35 years of singing, I have never encountered a song that I could sing if it were shifted a 4th or a 5th higher or lower in pitch than what I asked for. That would be too much for most singers, unless you are talking about a song with a very narrow tessitura, maybe a melody where every single note falls within the range of a fifth. There are very few songs like that.
