About ¼ or a fourth of the way, you can stop at Borgarnes, Iceland. This is almost an hour north of Hafnarfjordur.
About ⅓ or a third of the way, you can stop at Hvanneyri, Iceland. This is about 1 hour north of Hafnarfjordur.
About ⅔ or two thirds of the way, you can stop at Hvammstangi, Iceland. This is about 2½ hours north of Hafnarfjordur, or a little over 1 hour southwest of Saudarkrokur.
If you drive straight through without stopping at all, the total distance is 295 km. The distances shown above don't include local roads to get into each town, so if you're planning a long road trip, make sure you factor in traffic and road conditions, rest stop breaks, and interesting detours along the way. If you're in a rush, you can do the trip with zero stops, and your total travel time would be around 3 hours, 35 minutes.