Upvote:1
Qatar Airways checked baggage allowance, as with other airlines, goes by two factors, weight and maximum dimension.
As example, for an economy class passenger (varies by Qatar destination), the total allowed weight is 30kg (66lb) with a maximum case dimension of 300 cm (118in). Say an average hardshell guitar case has an exterior length of around 43 inches/108 cm and weighs about 9 pounds/4 kg empty, the guitar adding 6-10 pounds/3-5 kg. The checked case should be well within what is allowed and, only if any other checked baggage puts you over the total weight limits, would you have to pay excess fees.
See Qatar excess bagggage rate, as they vary according to a number of factors such as origination/destination, whether they're purchased online or offline (airport, city office, call centers, check-in desk), etc.