AAC blocks are supposed to be treated like vitrified tiles for their level of moisture absorption and time for curing. Using a standard 1:6 mortar would work just as fine as long as any hammering needs to be done for chasing or cladding building services.
Though different companies manufacture and sell AAC block adhesives, which may cost about ?650-900 a bag, this question asks for a mix ratio, so I'm going to give one.
If you use a coarse (double washed) M sand and have access to good quality OPC cement, a ratio of 1:2 to 1:3.3 would work. Unlike tile adhesives, exposed cement faces are prone to thermal contraction and early dehydration; hence, add a styric butadiene rubber cement additive. Or any other polyester/rubber based additive that improves the elastic properties of the cementing agent. My suggestion would be Fosroc SBR latex or Zydex's EB50, and no none of them sponsored this answer. You'd find equivalent chemicals in Sika, BASF in various names, but they're all ultimately the same. Dosage may vary from brand to brand, and so does packaging, cure, and set times. Make sure to use optimal amounts of these additives as they may affect the final adhesion of mortar plaster to the blockwork.
clockwork However, maintaining a uniform grade of mortar or constant w/c ratio at the site is tedious, so a premixed bad might come in handy. The thickness of the application may not exceed 5-8mm.
The consistency needs to be like a paste(obviously) easy to apply using a notched trowel. If you're using a thicker bonding layer, your mix might end up cracking and may require additional microfibers to stick together. Water cement ratio of about 33-40% should work. Depending on your curing and additive, this could reduce to up to 28% nothing more.
Setting and curing
AAC blocks have significantly less initial water absorption. However, some water will be lost during the initial set. Adequate curing for 3-7 days is advisable. May vary depending on the use of additives and water content but curing never does any harm, so do them anyways. The maximum height to be built per day should be curtailed to four-course at any cost. Do include a stiffener beam (these are curtain walls anyways) every four feet for a 4? wall.
I strongly discourage the idea of using AAC blocks in confined masonry without columns. As much as I've researched, they cannot be used as structural walls, especially for multi-storied buildings. Ideal to be used in framed structures as a partition or curtain walls. They're good enough to hold cladding but not structural cladding like heavy hoists or lift counterweights.