If you want a natural solution, try lemon eucalyptus oil.
Considering that B1 does not deter mosquitos, any dose you like will be comparable to 75mg (zero effect). However, if we wish to assume it works, you'll want the patch. Eating B1 won't help much. Since B1 is principally excreted in your urine, eating a lot of it would only really help if you bathed in your urine afterwards.
New England Journal of medicine
Most alternatives to topically applied repellents have proved to be
ineffective. No ingested compound, including garlic and thiamine
(vitamin B1), has been found to be capable of repelling
Studies of garlic and vitamin B did not find evidence that these
substances could reduce mosquito attraction.
In the last few years, nonchemical repellents worn as skin patches and
containing thiamine (vitamin B1) have arrived in some big-box stores
under the name Don’t Bite Me! The science behind this repellent comes
from a study done in the 1960s. It showed that thiamine (B1) produces
a skin odor female mosquitoes don't like. But no other studies have
confirmed thiamine's effectiveness as a mosquito repellent when worn
on the skin. Chari Kauffmann, president of the company that sells skin
patch called Don’t Bite Me!, says studies on the product are ongoing,
though the company has no conclusions to report.
Generally if a study can't be confirmed within 50 years, including by a company trying to profit from it, it's a sign that there was a flaw in that study. In addition multiple studies (My apologies for some of the studies not being public access) have found the counter, which is that is is not effective.
You should also be aware that everything is a poison at some point. In the case of B1, long term dosing (like say, frequently to ward off mosquitoes) of 3g or more could cause harm.