How long does it take to fully charge car battery by driving?

You're going to have to do quite a lot of driving to fully charge a completely discharged car battery! That's a fact. You really need to have your battery charged with a battery charger. Go to one of the major auto parts stores and ask them if they do that or, if not, where you can get it done. They will be able to tell you that,, add they are familiar with the local auto services. As for you leaving your car parked, and idle for three weeks, it should start OK as long as the battery is Good, and the weather isn't extreme. Hope this helps! Good luck!

With a proper 3 stage charger it takes about 10 hours to fully charge a lead-acid battery. With your alternator the time required will vary, but it will always be many hours for a full charge.

I mostly agree with everyone so far: Driving 20-30 minutes will surely charge the battery enough so it will restart tomorrow or for a few days..............but it probably won't be fully charged...........and you ARE putting an extra load on the alternator. I'd suggest getting a small battery charger (like the Battery Tender Jr) that only costs about $20, you can leave on overnight to slowly charge the batter to 100% ( a slow charge is preferable to a high-amp quick charge). New batteries amaze me - I'm used to old-school batteries that needed to be replaced every 4 years. We now have a 2006 car at our vacation home. We have periodically run it completely dead (factory theft system drains it in a couple days and sometimes we accidentally turn it on)...................we put the charger on it. Original battery now 7 years old - we returned to the vacation house after the car sat 6 months - and it started right up - WoW - so your Accord battery should bounce back (btw - I really like Hondas - specially the new Accords. BUT our fine Ford products automatically turn-off accessories and lights after 20 minutes if left on accidentally - just wish there were a way to disconnect the anti-theft system).

A half hour will charge a battery back up. A four-year-old battery? That is about all the life you will get out of it. Get a new one. Lead-acid batteries last two to five years. Longer if they are kept charged, clean, and serviced. If allowed to discharge or get low on electrolyte, or left idle for long periods of time, death rate accelerates. I have tried everything to revive old batteries: slow charge, wash them out, replace electrolyte, keep them warm, etc. Regular maintenance and constant use keeps them running the longest.