Red[
    author: {Nędza Darek}
    version: 0.0.2
    license: {
        - point to this gist/github
        - no warranties
        - use/modify everywhere
    }
]
float-incr: function [v] [
    digits-after-dot: length? find/tail (to-string v) #"."

    s: copy "0."
    loop digits-after-dot - 1 [
        append s #"0"
    ]
    append s #"1"

    v + to-float s
]
random-loot: function [chance [float!]][
    if chance > 1.0 [do make error! "chance bigger than 1.0"]

    digits-after-dot: (length? to-string chance) - 2 ; "0."
    chances: make block! N: 10 ** digits-after-dot

    chance-win: to-integer at (to-string chance) 3
    chance-fail: N - chance-win
    ; thank you @rebolek for reminding me about loop
    loop  chance-fail[
        append chances 'fail
    ]
    loop  chance-win[
        append chances 'win
    ]

    random chances
    random/only chances ; pick one
]

state: 'fail
chance: 0.001
number-of-tries: 0
while [state = 'fail][
    number-of-tries: number-of-tries + 1
    state: random-loot chance

    if 0 = mod number-of-tries 100 [ chance: float-incr chance]
]
number-of-tries