Where to put savechanges in unit of work pattern with asp.net mvc?

Yes,you have to do it inside the UnitOfWork.cs class.

UnitOfWork.cs

 public sealed class UnitOfWork : IUnitOfWork
   {

       private DbContext _dbContext;

       public UnitOfWork(DbContext context)
       {         
          _dbContext = context;
       }

       public int Commit()
       {
           return _dbContext.SaveChanges();// Save changes 
       }

       public void Dispose()
       {
           Dispose(true);
           GC.SuppressFinalize(this);
       }

      private void Dispose(bool disposing)
       {
           if (disposing)
           {
               if (_dbContext != null)
               {
                   _dbContext.Dispose();
                   _dbContext = null;
               }
           }
       }
   }

After that you can call UnitOfWork's Commit method inside the Service Layer.

EntityService.cs

public abstract class EntityService<T> : IEntityService<T> where T : BaseEntity
   {
       IUnitOfWork _unitOfWork;
       IGenericRepository<T> _repository;

       public EntityService(IUnitOfWork unitOfWork, IGenericRepository<T> repository)
       {
           _unitOfWork = unitOfWork;
           _repository = repository;
       }  

     public virtual void Create(T entity)
           {
               _repository.Add(entity);
               _unitOfWork.Commit(); //saving point        
           }

}

You can learn more about this pattern : Generic Repository and Unit of Work Pattern