Steps to reproduce:
1. configure cups to use 2 network printers one that is in the local network (ping'able and will print just fine) and one that somewhere far away but neither ping able nor connectable however the name of that remote printer must be DNS resolvable.
2. open a gnome application and open the print dialog
3. in the gnome print dialog select the remote network printer

-> the whole application will hang


Stack trace:

+ Trace 185025

$5 = (GSource *) 0x84f0200
(gdb) p timeout_
$6 = (gint *) 0xbf9c4ce8
(gdb) p *timeout_
$7 = -1
(gdb) p *source
$8 = {callback_data = 0x83da268, callback_funcs = 0xb75499a8, source_funcs = 0xb5944860, ref_count = 2, context = 0x8105018, priority = 0, flags = 1, source_id = 529, poll_fds = 0x0, prev = 0x83b4900, 
  next = 0x83b4608, reserved1 = 0x0, reserved2 = 0x0}

Interestingly the default was 5 seconds!

(gdb) p timeout
$11 = 5

Looking at the code

static gboolean
cups_dispatch_watch_prepare (GSource *source,
			     gint    *timeout_)
{
  GtkPrintCupsDispatchWatch *dispatch;

  dispatch = (GtkPrintCupsDispatchWatch *) source;

  GTK_NOTE (PRINTING,
            g_print ("CUPS Backend: %s <source %p>\n", G_STRFUNC, source));

  *timeout_ = -1;
  
  return gtk_cups_request_read_write (dispatch->request);
}

timeout is set to -1


So to me it seems setting it to 5 seconds or so should do it?