To use a timeout value other than 30 seconds, replace

myStore = new Ext.data.JsonStore({
  url: "http://foo.bar.com",
  ...
});

with

var myBigTimeout = 90000; // 90 sec
myStore =  new Ext.data.JsonStore({
  proxy: new Ext.data.HttpProxy({ url: "http://foo.bar.com", timeout: myBigTimeout }),
//url: "http://foo.bar.com",
  ...
});

ExtJS