if you want to apply for a China visa in Singapore, you shall provide documents that entitle you to reside for an extended period of time in there, such as a PR, WP, EP, STP, DP. If you are a temporary visitor to Singapore, then, you should apply for a Chinese visa in your own country.
generally, you will be issued a visa with the duration of 30 days. but 6 months F or L visas are possible to get if you can provide sufficient proof that you need to visit China frequently during the half a year. In addition, your previous China visas are helpful if you have any.