The is no official rule for how long you will be given on re-entry to the US after 5 months in Mexico.
However US Customs and Border Patrol do use a rule-of-thumb that you should be outside of the US for longer than you are in the US. So after a stay of 6 months, followed by being outside of the US for 5 months, that rule of thumb would mean you would likely be admitted to the US for a further 5 months - although given that's close to the normal B1 visa limit of 6 months it's very possible they would simply give you a further 6 months on this occasion.
HOWEVER this is based on the presumption that they find you to be a bona fide tourist. Staying in North American for over a year would not be normal "tourist" behavior, and it's very likely that you will be question regarding your intentions whilst in the US, as well as to what you did in the US on your previous trip and what you did in Mexico.
If the CBP staff believe that you are attempting to 'live' in the US, or that you will potentially work whilst you are there, then it is very likely that you will be refused entry into the country, or be given a much shorter entry period.