Hey Guys,
I want to ask, if I do long threads or replies on forums or Q&A sites and having my links under them (signature or source link), does it have any SEO benefits?
Not really
or just starting with one or two lines, It's enough for a post out there?
Also No. If you mean just post one liner forum posts to get your signature to show up, you may be in a shorter forum thread which may have slightly better SEO benefits, but consider the human element also.
If your business is in xyz niche, and you spend time on xyz forum. Even if you do not get any SEO benefit from anything you do there, people will notice "hey, the guy from Acme xyz is really knowledgable and always helps out, let check them out". If you just post one liners to make your signiture show up more, people will think, "There's that guy from Acme xyz spamming the forum again, what a &%#@"
Quality content is always #1. Whether it is content on your site, blog, other sites, forums, etc, etc, etc. Spend the time, put in the hard work, provide quality content and you will succeed in the end!