Tip life is inevitably worse with lead-free solder. Using a brass wool cleaner rather than a wet sponge will help prolong tip life, as will the regular use of a suitable tinning/cleaning paste (e.g. Hakko FS-100 or JBC TT-A). Keep the tip wetted with solder as much as possible - a completely "clean" tip will oxidise much faster than one with a protective layer of solder.
The correct tip temperature for any hand soldering operation is the lowest temperature that will allow the joint to be completed in two to five seconds. In practice, that depends on a host of variables - the composition of the solder alloy, the properties and calibration of your iron, the thermal mass of the joint etc. A usual rule of thumb is the melting point of the solder plus 150°C, but your mileage will vary.
The correct tip temperature for any hand soldering operation is the lowest temperature that will allow the joint to be completed in two to five seconds. In practice, that depends on a host of variables - the composition of the solder alloy, the properties and calibration of your iron, the thermal mass of the joint etc. A usual rule of thumb is the melting point of the solder plus 150°C, but your mileage will vary.