I have a Crowley-esque character (the demon from Supernatural) who is concerned about ehem certain features, and this is exactly the kind of petty nonsense this character would do. I'm looking for a way without endless scrolls that the duration of Alter Self could be cast only once or twice a day, preferably that could take out the concentration component as well.
The closest you can get is with the warlock invocation master of myriad forms, that lets you cast alter self at will. It still requires concentration and you have to refresh it every hour. It also needs you to have at least 15 warlock levels.
If levels are at a premium, but only the illusion of a different shape is enough for most of the time, you can go with mask of many faces, that requires only 2 warlock levels and lets you cast disguise self at will. If you take another 3 levels in sorcerer, you will be able to sustain alter self for 4 hours with the extended spell metamagic.
If you are the DM making an NPC, I recommend you just grant them the spell at will or base them on a shapeshifter fiend. It is unlikely to unbalance anything and might add nice flavor to the character. Also, neither at will low level spells or shapeshifting is rare in fiends, so you could consider it an "established mechanic". As far as that can go for a custom NPC anyway.
This is going to be a tough thing to achieve, and some will argue that you cannot do it in the way that you would like. I say you can, but I bet I get comments otherwise.
Once or twice a day can, AFAIK, only be achieved by having the Sorcerer Metamagic Extended Spell. You'll need to get DM buy-in on this, as it isn't fully clear, but I read Extended Spell as allowing multiple Sorcery Points to be spent to extend a spell, up to a maximum of 24 hours. If your DM agrees, then you will need to take 4 or 5 levels of Sorcerer, take the Extended Spell Metamagic, and cast Alter Self using 4 Sorcery Points for a 16-hour duration (and hide in your coffin the other 8), or 5 Sorcery Points for the full 24 hours.
Removing the concentration component is more difficult, and will require some effort and preparation. You will need to cast a Glyph of Warding (which takes an hour to cast, and is not mobile once cast) with the extended Alter Self in it, then trigger it to cast the spell on yourself. Being able to do this will require taking 5 levels of Bard, Cleric, or Wizard.
The other option, which is kind of close to the effect you want, is to be a Warlock and get the Invocation of Mask of Many Faces at 2nd level, and switch it for Master of Myriad Forms at 15th level. This method is straightforward, but would still require concentration. That would let you operate a normal life with a different appearance, but would risk dropping that appearance under certain conditions.