Decay is applied on open positions every 8 hours as counted from the time they are opened. Decay is an amount that is subtracted from your active P/L and added to the liquidity pool (not a fee paid to WCX).
Decay can be seen as a small charge on unrealized P/L. Other margin trading platforms call it 'funding', 'overnight premium', or 'swap'. The difference is that decay is not applied at a specific time for all positions – it's applied after 8 hours on each individual position, which is more predictable and advantageous to traders.
If you open a 10 BTC position on a market with a daily decay rate of 0.01%, in a 24 hour period, the decay would be:
1 * 0.01% * 10 = 0.001 BTC
Meaning every 8 hours, the decay charge is:
0.001 / 3 = 0.000333 BTC
if you keep the position open for less than 8 hours, no decay is applied.
if you close the position within 8-16 hours, 0.000333BTC decay is applied.
if you close the position within 16-24 hours, 0.000666BTC decay is applied.
Decay rates for each market can be found here.
The profit and loss numbers displayed in your dashboard already take into account any decay that may apply to your position.