When will Howard Wolowitz get his PhD?

Considering the real world requirements to achieve a PhD, then yes it's realistic that it will take Howard 2 to 3 years (each season is one year roughly) to gain a doctorates degree.

See this answer to Does a masters degree help completing PhD sooner? for some details on what's required. In short, it takes meeting the Universities requirements, courses, and research. It's not a fly by night certificate. Howard's work would not necessarily count as course credits or anything, but might depending on the University or his Doctorates mentor.

The show has a habit of not explicitly mentioning all the background efforts of the individual cast unless needed (See the question about Penny's pharma rep job), so it's safe to say that Howard is working in his PhD until we are told he stopped or until he completes it.

in Season 8 episode2, "The Junior Professor Solution", Sheldon becomes Junior professor.

According to this site:

Howard volunteers to take his class since he wants to start work on his doctorate.

Maybe he his secretly still working on it and in some later season we will have a surprise... or not: it is fun to pick on him, I am not sure authors want to lose an easy chance for some gags.

In real life Howard would earn the highest income of any of his friends, save Penny. With sales commissions, Penny's income could be much higher. Howard's position is not a teaching position so a doctorate would not be required. Maybe he doesn't have a desire to teach or move. So based on personal experience there is no reason to get his doctorate except personal satisfaction. After 6-8 years of experience any increase in salary Howard would get by having a doctorate would not justify the expense of obtaining one.