Cakes made of whipped-cream by professionals last for a long time in a consistent, white state. However, from what I know, whipped cream tends to become yellow and return to a more liquid state in a few hours. I prepare whipped cream with just the cream and an electric mixer, so I assume there's some magic ingredient or process to keep the thing stable. Where's the trick ?
There are two levels to this question. If you are using modern whipping cream as a starting point, then the trick for a longer lasting whipped cream is stabilization. If this is some farm bought milk you are skimming the cream from, then you have the additional problem that your cream is not fatty enough. Modern creams are concentrated with a centrifuge.
To get good whipped cream, whip it cold until it doubles in volume and you get firm peaks. Stabilize the whipped cream by hanging it in a cheese cloth in the refrigerator or by adding gelatin.To use the gelatin, dissolve 2 teaspoons of unflavored gelatin into 2 tablespoons of cold water. Work out the lumps. After the gelatin expands, mix in a quarter cup of cream and heat the mixture to dissolve the gelatin. Cool over ice and mix a bit. Complete 2 cups of cream and proceed as you would to finish your whipped cream.
For stable frosting, I whip heavy cream and cream cheese together (start by whipping the cream cheese, add twice as much cream to it, slowly). Lasts days in refrigeration. Slight tangy taste though; whether that's good or bad is up to you...
There's a few things that you can add: agar-agar, cornstarch, powdered sugar (which has cornstarch), gelatin (requires blooming in water first), powdered milk, etc.
Search online and you should find the proper ratios of each one to use so you don't overdo it.
bloom some geletin (flavorless! do not just grab a packet of jell-o) and add it to the cream.
for 1 cup of cream 1/2tsp of geletin in 1Tbls of water.
Real cream with a high fat content + a hardening agent. You can buy it in any supermarket. Just mix it with the cream before whipping.