You don't tell us how big the building is, but let's makes some assumptions. 100 bodies, average family size in the U.S. of 2.6 people. That's 38.46 households. Let's call it 40 (couple of single people...). Average size of an apartment? Let's say 1,000 sqft. You need space for stairs and halls. That's a 50,000-60,000 sqft building. How many stories? It can be one to ten. Let's say six (10,000 sqft/floor? let's say ten). That means stairs, doors, carpet... You could use sheets to transport the bodies, but the fact remains that they've sat there decomposing for two weeks.
What's the point of cremating anything when the building is biologically more dangerous than leaving the bodies outside to finish decomposing?
But that wasn't your question... You have five adults. Let's ignore the gender breakdown and assume they're all equivalent workers. My wife has more endurance than I have, I can lift heavier things than she can. The two of us tend to get an equivalent amount of work done before we're pooped out. We're athletic in the way that active people who eat chinese food and BBQ tend to be. So, how long would it take she and I, and three friends, to clear that building?
I can believe we can clear the building in 2-3 days. Probably less. How old are your adults? You don't say. Five people like my wife and I wouldn't be in the mood to collect firewood after days of climbing stairs in a ten-story building and hauling decomposing bodies outside. Realistically it would probably take us a week. But if we put our minds to it... OK, three days.
Now, combustibles. I found the same references @CauêMoraes did. Let's assume 500Kg or about 1/2 ton of wood can cremate a body. That's 50 tons of wood. Gasoline can help start the fire, but it burns hot and fast. Fast is the problem. So it won't help. Fuel oil would help, but that's only available in specific areas (like the East Coast, none at all where I live, we use propane. Don't use propane.). So, wood. 50 tons of wood. By hand. 10 tons of wood each. By hand. (I've demolished buildings by hand... Ugh.) We don't live in an area where there's a ton of trees within walking distance of town. So we're demolishing houses and buildings to get wood. Well, we could get some of it from our local hardware store or lumber yard. But now we're hauling wood from a distance. We have gasoline, so maybe we have trucks? That would get us a bunch of wood. Is there a Home Depot nearby? That would get us more wood. If we have trucks and local lumber yards and/or hardware stores then 50 tons of wood isn't that hard to get. If we don't... It would take a month, minimum, maybe six months to demolish by hand enough houses to get 50 tons of wood. That's an outrageous guess, but I'm sitting in my chair staring numbly at my computer screen just thinking about it. So, trucks and a Home Depot, another day. No trucks or Home Depot? Months.
And let's not ignore that fire needs boatloads of oxygen to burn. Lumber is terrible when it comes to that. All those lovely square objects that minimize oxygen flow. Tree logs would be much better. But we're likely stuck with lumber. All I'm saying is you can't just borrow Home Depot's forklift and set down a bunch of stacks of lumber, throw the bodies on top, douse with gas, and let it burn. It won't burn that well. It might not burn substantially at all. You need slow and hot to cremate a body. Slow and warm won't do it. Slow means lots of fuel. Hot means lots of oxygen. The lumber needs to be carelessly tossed into a heap to get the oxygen flow or stacked up in log-cabin-like grids. Time. Time, time, time, time, time.
Another day to stack all the wood. Maybe two. We're in our 50s.
Once it's ready to burn we need to wait for good weather. Let's just ignore that. Then we need to "borrow" some generators from Home Depot and more of that gasoline along with some Rug Doctor floor cleaners to thoroughly clean and disinfect that building to make all the effort we put into burning the bodies worthwhile...
Wait a minute...
You have gasoline?!!!
You have gasoline. Why on earth would I waste one second of my time trying to cremate anything? I'd look around for a backhoe or a Bobcat and a dump truck. I'd put all the bodies in the truck (do I even need to haul them down stairs? Can I throw them off balconies?), drive the truck downtown, and leave it. I might not even bother to turn it off. Don't want fuel in the truck for some prankster to drive it back because they don't want the bodies in their neighborhood. If I'm really being a good citizen, I take the dump truck to the dump and, well, dump the bodies. The truck might come in useful later.
If I have access to gasoline, there are a thousand other ways to solve this problem that are easier than cremation. And since the city is filled with buildings full of dead bodies, what would be the point of cremation other than to ruin a perfectly good field or street with the ash and heat?
Cremation: weeks to months depending on how hard it is to get the wood.
Any other solution: A day. Maybe two if you have to find the dump truck.