throttler.cs

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        static void Print()
        {
            Console.WriteLine($"Invoked at: {DateTimeOffset.UtcNow}");
            Thread.Sleep(500);
        }

        static void Main(string[] args)
        {
            var throttler = new Throttler(TimeSpan.FromSeconds(1), 5);
            for (int i = 0; i < 10; i++)
            {
                Thread.Sleep(100);
                Task.Run(() =>
                {
                    while(true)
                    {
                        if(throttler.IsOk()) Print();
                    }
                });
            }
            Console.ReadKey();
        }
    }

    class Throttler
    {
        public Throttler(TimeSpan interval, int maxRequests)
        {
            Interval = interval;
            MaxRequests = maxRequests;
            _timeStamps = new DateTimeOffset[maxRequests];
        }

        public TimeSpan Interval { get; }
        public int MaxRequests { get; }

        private readonly object _locker = new object();
        private readonly DateTimeOffset[] _timeStamps;
        private int _lastIndex;

        public bool IsOk()
        {
            lock (_locker)
            {
                var now = DateTimeOffset.UtcNow;
                if (now - _timeStamps[_lastIndex] >= Interval)
                {
                    _timeStamps[_lastIndex] = now;
                    if (_lastIndex == MaxRequests - 1)
                    {
                        _lastIndex = 0;
                    }
                    else
                    {
                        _lastIndex++;
                    }
                    return true;
                }
                return false;
            }
        }
    }

}