Amber14sb has the parambsc0 description for DNA and RNA. So it is a good choice.

Please check the link

"Molecular Dynamics Simulations of Intrinsically Disordered Proteins: Force Field Evaluation and Comparison with Experiment"

http://pubs.acs.org/doi/pdf/10.1021/ct501178z

There is no 'perfect forcefield' . And simulation time depends on many parameters such as your interest of the problem and your computational resources. 

Thanks everyone for replying.

@Sarath Chandra Dantu, even i have read about ff14SB force field to work on nucleic acid but i am not sure if i can use it for an IDP, as they are more flexible than structured protein. 

You can check this paper. It reports on a new forcefield for IDP's 

https://www.researchgate.net/publication/309762602_CHARMM36m_an_improved_force_field_for_folded_and_intrinsically_disordered_proteins

Article CHARMM36m: an improved force field for folded and intrinsica...

Thanks :)