I submitted an scientific article to the scientific journal "High temperature materials and processes" hosted by Walter DeGruyter Publisher and it is Under Review. I want to know how many weeks it take (Average) in the peer-reviewing process.
There's no such thing as an average time in then matter of peer reviewed journals. It depends on whether experts in the field are available to do the job, whether they're willing, or are they on holiday now? All sorts of things....
As a submitter, you have no real rights to expect anything in particular. You are a supplicant, not a service user. You should always check in advance what to expect from any particular journal by asking colleagues. RG offers a good way of doing that.