estimated around 1hour, but normally less than that. But to be safe, should allocate 1hours or more.
There's 2 Rapid bus heading toward airport, 1 of it is express, direct to the airport.

Plenty good food along that street and the side streets like Cintra St.  At the junction of Leboh Carnarvon and Chulia St., there are many stalls that opens at night on the street. I like the 'Kway Teow Th'ng there. There's also a nice nasi kandar at the junction with Penang Street (not Penang Road). There is a good beef noodle at the coffee shop in front of the Bomba  at the junction of Beach St. I think you should walk along those streets, Campbell  St. where Hemeediah ( most famous for it's nasi bryani ) is.  Cintra St. also has many good 'makan' In fact Line Clear nasi kandar is just at the start of Chulia Street. I don't think you will be able to actually savour all the good food on that street. Be adventurous and walk around that area.

Anyone here from Penang? I would like to know if bus service frequency stated at RapidPenang website is reliable?

At the airport just ask the bus driver if any bus going to Pragin Mall, if not you have to take the bus to komtar bus station to take the bus that pry Pragin Mall. At komtar bus station you can go to anywhere in Penang island. There are also free shuttle bus there so just check it out at komtar bus station, you won't get lost.

Anyone know wat bus to take to go to Queensbay Mall frm Komtar?