I'm working for about 12 hours a day, I don't really need so much work to live (that is two working journeys). Accordingly to Islam, is something recommended or not to someone overwork?
There are many narrations about working:
Ali bin Abi Hamza says: One day I saw Imam Musa ibn Ja'far (AS) worked on his farm, and because of hard working was engulfed in sweat from head to toe. I said: Where are the workers that unless you're working on?
He said: Who better than me and my dad had his hands on his farm worked! I said who was he? He said: The Prophet (SAW) and Ali (AS), and all my ancestors worked with their hands, and this is the behavior of the prophets and their successors and righteous servants of God.
The Prophet says:
one who provide his livelihood by working and efforts , in the qiyamah will be placed among prophets and will get their reward. source:mostadrakol vasael , volume 2, 8th bab.
But there are some narrations about the time of working as recommendation:
Imam Ali ibn abi Talib , nahjol balagha:
للمؤمن ثلاث ساعات: فساعة یناجی فیها ربه، و ساعة یرم معاشه، و ساعة یخلی بین نفسه و بین لذتها فیما یحل و یجمل و لیس للعاقل ان یکون شاخصا الا فی ثلاث: مرمة لمعاش، او خطوة فی معاد، او لذة فی غیر محرم
The faithful must split his day's times into three part : one for praying Allah, one for working and gaining livelihood and one for himself and halal pleasures.
Islam puts some responsibilities on you with respect to Allah, your parents, siblings, children and other people. If you are fulfilling these responsibilities, you can work as much as you want. Consider that your body is also your responsibility and you have to take care of it too.
In terms of hour, it depends on person to person and thus left at personal discretion.