What are the criteria for re-combining?

After you launch half your mass, it seems that you can't immediately re-combine your separate masses into one bigger cell.

Is this based on some amount of time, or some other criteria? Exactly what do I have to do before I can re-combine back into one big mass?

Solution 1:

The time to recombine is based on the size of the offshoots. When the cell is split, every individual cell has a timer storing how long until they are able to recombine. There may be a component that if you have cells much smaller than your largest cell the timers for the small cells are reduced.

Edit: So having looked through the source code at the link @BitCoder provided, I had it partially correct. The server has a base recombine time, currently 30 seconds. When a new cell is created (either via the player splitting, or running into a virus), the server sets the recombine time to be:

30 sec + (.02 * mass))

In summary: the recombine time of each cell is thirty seconds plus one additional second for every 50 mass the cell has when it is created.