As has been mentioned, I would advise dropping tab (or use VERY sparingly). Instead train your ear by playing along to simple songs and working out the bass line yourself by trial and error. Yes, it is hard work and takes longer than tab, but in the long term the benefits are huge.
As for practicing and for how long: regularity is more important than duration, as is WHAT you practice. Half an hour a day EVERY day is better than three or four hours at weekends.
As for WHAT to practice. Go to the study guide in the link, start at the beginning, and slowly work your way through the lessons as they occur.
Study Guide | StudyBass
All good advice given so far, but I especially agree with what fearceol said, "Half an hour a day is better than multiple hours on the weekend."
This is how I practice: I try to practice for at least 20 minutes to more than an hour almost everyday.
I say almost everyday, because I find that if I practice four or five days in a row, then take a day or two off, my playing seems to get a little better after the break.
And, I play the same boring portfolio of songs until I get them perfect.
I've been at songs like, "The Lemon Song" by Led Zeppelin, and, "Spain," by Al Jarreau.
I also practice some standard Blues and Jazz patterns until I want to scream, but I keep at it for two reasons: I love music, and playing, and I read that as a person ages, two things that help keep the mind sharp are, crossword puzzles and playing a musical instrument.
Next on my list: I'm going to try, "Rhythm Stick," by Ian Dury.
One of the hardest and fastest bass lines I can think of.
Keep at it, and never give up.
Give yourself a break every now and then.
Good luck.

Mike