From the international body http://www.fina.org/content/coaches-certifications

My daily mileage is around 1.6-2.2k depending on how much I get done in the (40-50 mins) morning before work and I also get very cranky on my work trips with crappy short hotel pools. If it's rectangular it's a bonus - the worst are those novelty pools shaped like a figure 8...

Having said that if you're doing that type of mileage, you should have a good idea of your pace zones and you should be able to swim according to your perceived exertion. So just swim based on feel - your pace/100m is not going to be terribly useful so just use it to keep your fitness up.