Coin maturity (staking symbol turning green) is reached exactly 24 hours after first deposit. When you get first reward (aka - mine your first block) depends on how many coins you have, and how many other coins are staked by someone else (thus the function of "your weight" vs "network weight".
If you withdraw some ECA, it depends. You can only spend whole inputs (stacks) of coins - but if you send them in several lots, you should manually select the input that is closest to the amount you want to send - then only this position is reset, and the change starts from zero - all the other positions remain unaffected.
Is this from the ECA wallet or from CoinsMarkets?
Thanks!
The above is if you are staking with your own ECA-QT-Wallet (your system)
If your leaving your coins at Coinsmarkets, it doesn't matter what you do. Coinsmarkets collects all the staking rewards that have gone to addresses in the Coinsmarkets Wallet (that of the Exchange, not the local accounts of their individual customers), and distributes them every 24 hours proportionally among all the customers having balances at the time of the distribution (normally after 8.12 pm UTC). Your share is proportional to the number of coins you hold at that moment, and not of the average during this period.
Due to the high transaction volume on coinsmarkets in the last few days, the time the coins actually rest at a given address has likely much declined, and thus the share of coins staked at coinsmarkets might be comparatively low (compared to HODLers). Thus, the actual reward distributions are relatively low, compared to earlier times with very little trade and transfer activity...