If you're using a good lube like Dumonde Tech Lite, which is what I use and it last a good 500 miles, Rock N Roll Gold or Ultimate Dry can go at least 250 miles; ProGold ProLink will also last at least 250 miles. Those 3, and especially the Dumonde Tech Lite, I have found to last longer than any other lube I've tried, which is quite a few, but obviously I've haven't tried them all.
All I ever had to do with one of those three lubes is to wipe down the chain after every ride including the jockey wheels and a brush for the gears or gear floss, I would not apply more lube till I hear a bit of noise from the chain, applying it every week is an overkill and unnecessary unless you're using one of the liquid waxes in a bottle crap. The OP is only riding 40 miles a week, if he uses one of the Lubes I mentioned he won't have to relube for a long time as long as he wipes the chain down after, or before, every ride. Rain can be a problem, but Dumonde Tech Lite and the non-Lite version will hold up against rain extremely well with no need to reapply after several rain rides, as is Rock N Roll Gold. Now Rock N Roll Ultimate Dry can wash off in rain, but I've used and after a rain there was no noise from the chain so I wiped the chain down and used it the next day without relubing, no problem, but it is the only lube on my list that will not take a second rain and you will have to reapply it when you get home. Dumonde Tech is the most popular lube in touring circles because of the reasons I listed, and I know it works.
Dumonde Tech is a different lube, when you go to use it for the first time you have to clean the chain, then apply the lube as per it's instructions, then ride it for about 50 miles, come home, wipe the chain down after each ride, reapply the lube and ride again for about 75 miles wiping down the chain after each ride, reapply it for the last time for a long time, don't reapply till the chain just starts to make noise, which is true with all lubes. I've even washed the bike and still was able to ride the bike without relubing! Dumonde Tech is unusual in the lube industry because it's a type of polymer lube and puts a thin layer of "plastic" like coating on the chain and gears which is why the first 3 times you use it you use it in fast succession, which is why it last a very long time.