That's depend on your systems state, if you have all networking and storage configurations in place and just need to standup 10+ ESX hosts, than that won't be long. You can average installation of ESX 3.5 between 10-15 minutes and then general configurations with networking and storage would be another 20 minutes that's depends how your engineers knowledge with VMware/configurations themselves. I would spend 30 minutes per eSX hosts but if this is for consulting, than you would have to spend time on this types:
1. Meetings with the client and understand their needs.
2. Analyze their physical networks and systems
3. Write a Scope of Work (in details what will be done (architecture layout, project management & implementation phases)
4. Collaboration with other team (networking, storage, security, systems and application owners) takes time to do change management and actually deploy it.
5. P2V if anything requires for this purpose that takes time and planning.
6. Deploy ESX infrastructure (VC server, ESX hosts, networking and storage configruations and provision VMs and templates)
All of these steps takes time and requires good planning to have a smooth and secure environments, but generally if you just deploy ESX hosts takes about 30 minutes for all configurations. I wouldn't quote 30 minutes per hosts for consulting job 
since you charge by hour so spec it wisely.
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Regards,
Stefan Nguyen
iGeek Systems Inc.
VMware, Citrix, Microsoft Consultant