Am so sorry to learn about the loss of your beautiful Charlie.
Having lost two cats to road traffic accidents in the last twelve months, I am gradually changing from the "cats need to go out" camp to the "do they really" camp. I think I'm firmly of the view it's great for cats to have access to the outdoors where that can be achived practically and safely, but most houses these days are by, or near, roads. Living in the country doesnt necessarily make it safer for outdoor cats. I have a friend who lived in a wonderful rural part of Lancashire, and who lost three cats to road traffic accidents on her "quiet" lane before she gave in and cat proofed her garden.
A cat proofed garden would be my ultimate aim, but as has already been said - it isn't everyone who can have their garden cat proofed - cost plays a big part (not everyone can be a DIY expert, especially not me
) as does whether or not you live in rented accommodation (landlord's consent), or council regs (no structures over and above a certain height). all of those things can play a part in whether or not that's a suitable solution for you and your cat/s.
There are plenty of people on here who have indoor cats and whose cats lead very happy and fulfilled lives, so keeping a cat solely indoors is not "cruel" provided you're willing to provide the stimulation a wholly indoor cat would need - regular playtime, and plenty of things of interest, such as cat trees, indoor kitty grass, room to play and let off steam, a "window on the world....." etc to keep them active and interested in life.
Cats and roads dont mix. If you still wanted to go down the outdoor access route and didnt have the ability to provide a cat proofed garden environment, then you could consider getting two older cats - say 8 or above - they're less active (generalsing here - I've known some very active older cats
) and less inclined to roam huge distances. They would provide company for each other, and will probably want to sleep more and stay nearer their home turf.
However, you would always have to be prepared for that heart stopping moment when someone may call you to ask if you have a such-and--such cat, because if so, they have some awful news. Or facing day after day when they just dont come home and you never find out what happened to them.
My heart goes out to you over Charlie.
You'll know when it's time to take on another cat - and that's different for everyone.
There's no "right" or "wrong" time limit, just as Susanne says with regard to the thorny dilemma of "in" or "out" but you have to feel comfortable with whatever decision you make ultimately, and if that is to keep your cats indoors, then I'm sure you'll find a compromise to keep them happy and occupied.
There are so many loving cats out there just desperate for a good home, so dont ever feel guilty at the thought you're "replacing" Charlie - it's not a case of replacing your lost cat - you can never do that, for they're each one unique and individual and special - but you can open your doors and your heart to another who will reward you just as richly, albeit differently. And if you get two? Then that's double the pleasure.