First off, if you're new to WarThunder you should understand the currency system. Silver Lions are the in game currency used to repair vehicles and purchase new ones. You shouldn't have a shortage of silver lions until you get into tier ~4, so don't worry about stockpiling this too much. Next you have Golden Eagles, which is the 'premium currency' that is bought with real money and is used to make the grind easier. Then you have Research Points, which are used to upgrade vehicles and modules, as well as Free Research Points, which is what the game considers 'experience' in the leveling process. For this quest you want as much Free RP as possible, in the quickest amount of time.
The game mode that I've found earns the highest and most consistent yield of Free RP is Air Arcade Battles. In Realistic Battles, you can climb up to altitude for 20 minutes and not even get a kill, earning you less than 400 RP for a full game. But a mediocre game of Arcade with 2-3 kills will end in less than 10 minutes, and still grant you anywhere from 1000~500 RP depending on if you win or lose. Generally I've found all modes of Ground Battles to be less rewarding too, giving about half as much RP as you would get from Air Arcade, regardless of how well you did.
So since Air Arcade is the most beneficial, unless you're absolutely horrendous at flying, I'll give you tips on what to play for the best results in that mode. First off, Russian bias is very alive in the biplane tier. The USSR's I-15 is the best turn fighter in the game, and is unparalleled when doing vertical loops unless you are engaging another I-15. The guns on the I-15 have a low burst mass, meaning they don't do a lot of damage, but they fire extremely fast. This means you want to engage fighters that have weak and fragile wings and go for pilot sniping when engaging bombers. You are extremely fragile, as most biplanes are, so focus on playing defensively and don't chase onto someone when you think someone is coming up behind you.
Most people think that you need to move up to better planes to get a good return on Free RP, and they would be right. But what I would advise is to play all the tutorials, and get as much Golden Eagles from them as possible. This will be used to purchase a module called a "talisman" on two of your I-15's, which increases your return of Free RP by 100% when using that plane. You have to get all the way into the jet era to see a return of Free RP like that.
The reason you want to talisman your I-15's over the later stage aircraft is because the Talismans are cheap on biplanes (80 GE), and player skill is very poor at that level compared to Tier III and up players, which means you are extremely likely to at least come out in the upper half of your team in most games. Another way players can skip a portion of the grind is buying a premium membership for a day with the awarded Golden Eagles, but that only last 24 hours and gives the same reward as the talisman.
Lastly, winning is very important to maximize your RP gain. Winning a game could mean the difference between 1500 RP and 750 RP. Bombers usually decide who wins and loses in Air Arcade, so if you feel like you need to assure a win you can take out the SB 2M-100 and bomb armored vehicles, but you wont get very much from a non-talisman-ed bomber. So my best strategy is to only use the bomber in games where I performed exceptionally with my I-15's, or else it's a waste of time to go for the 200% bonus for winning, and would be better spent playing another match where I'm more likely to win.
Some other tips that will help carry you through the grind coming from a Warthunder player that used to play this game back in the dark ages of 2012-2015. Make sure to play around with some of the planes you unlocked or go into ground battles every once in a while. It's not as rewarding, but taking a break from the I-15 is refreshing. Second, don't head on anyone. You're more likely to ram into each other or get your engine shot out than actually scoring a kill, so just dodge their shots and get on their tail after you merge. Third, if your teammate ran into you, it isn't their fault. Only you can prevent getting hit, so if that means you need to pull off of someone's tail because your teammate is flying recklessly trying to kill the same person, do it. It's not worth losing you're plane, and it's very difficult to have good situational awareness when you are in a moth ball of aircraft.
I have 12 hours logged into WarThunder so far, and am at the start of level 10. I performed exceptionally because of the combination of my experience, enemy lack of experience, and the overpowered-ness of the I-15, and this put me in the top 00.3% of WarThunder players in terms of average score per game. If I didn't mess around with different tactics and just used the method that I wrote above, I firmly believe I could beat all three quests in <20 hours. Two hours a day should be fine for anyone that is new to WarThunder and wants to space it out instead of doing binge gaming sessions.