That is what I earlier mentioned, here in Qatar I think the validity of the Job Visa is tied to the visa approval from the government.
For getting an employee company should 1st request to the government that they want an (or a set of) employee(s) of so n so nationality to work for so n so profession and if government approves it, then only they can select a person according to that criteria and apply for his visa. So, as far as I know, the 1st approval has a validity and the visa issued using that approval is valid for that period, but I think, it can be extended by submitting another application. So best way is tell your new employer from Qatar about your 45 days notice and they will arrange provisions for that,
and when you have your visa you can see the validity (exact date till it is valid) printed on it clearly.